[enh] Add a specific documentation for Android build

doc/build_android.md

+# Android Builds
+
+Cesium can be build as Android App.
+
+## Prerequisites
+
+### Install build tools
+
+```
+sudo apt-get install build-essential
+```
+
+### Install libsodium 
+
+[The Sodium crypto library (libsodium)](https://download.libsodium.org/doc/installation/) is a modern, easy-to-use software library for encryption, decryption, signatures, password hashing and more. 
+
+- Get libsodium
+```
+    wget -kL https://github.com/jedisct1/libsodium/releases/download/1.0.11/libsodium-1.0.11.tar.gz
+    tar -xvf libsodium-1.0.11.tar.gz
+```
+
+- Installation:
+```
+    cd libsodium-1.0.11
+    sudo apt-get install build-essential
+    sudo ./configure
+    sudo make && make check
+    sudo make install        
+```
+
+
+### Install NodeJS 5
+
+Cesium need NodeJS v5. You can install it the [this download page](https://nodejs.org/download/release/v5.12.0/), but we recommand the use of MVN (Node Version Manager). 
+MV? help you to manage many versions of NodeJS. 
+
+ - Install [MVN](https://github.com/creationix/nvm):  
+```bash
+curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.31.1/install.sh | bash
+```
+
+ - Open a new terminal (to reload environment), then install NodeJS 5 : 
+```bash
+nvm install 5
+```
+
+
+### Install JDK (Java Development Kit)
+
+You can choose to install
+ 
+- on Linux, OpenJDK 8
+ 
+  * ``sudo apt-get install openjdk-8-jdk``
+    
+- or Oracle JDK 8 (all platforms supported) :
+  
+   * Download it from [Oracle web site](http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html)
+   
+   <img src="./fr/img/fef4f4dfe7c2168cb27c9e7f5e399fd547ce774a.png" width="400">
+
+   And choose the right version, depending on your platform.
+
+### Install Android Studio
+
+
+Download Android Studio from the [AndroidStudio-Downloads page](https://developer.android.com/studio/index.html#downloads)
+
+ - On MS Windows
+  
+  * Download file **without** the Android SDK:
+  <img src="./fr/img/3b8fa2f5c0465b13ae5ce74d49702e0c9f027866.png" width="690" height="237">
+  
+   * Then follow installation steps.
+
+ - On Linux : 
+ 
+  * download the full archive, then uncompress it (e.g in `/opt/android-sdk`).
+
+  * open a terminal, run the command:
+  ```bash
+  ./bin/studio.sh
+  ```
+
+At the end of the installation or the first launch, Android Studio will indicate that you do not have an SDK and will propose to install it.
+
+Install the version that it proposes to you.
+
+### Install Android NDK
+
+> The NDK is used for code execution in C ++, including the NaCL cryptography library.
+
+ - Download NDK from [this page](https://developer.android.com/ndk/downloads/index.html)
+
+   * **Warning**: Please use version `r10d` (not tested on newer versions).
+
+ - Uncompress the archive (e.g on Linux, in a new directory `/opt/android-ndk`).
+
+ - Launch Android-studio, then open the menu `File > Project Structure...`
+
+  <img src="./img/04e64b769cbd45b9d275cd5f81002a399a1a7684.png" width="300">
+
+ - A window like this should open: 
+
+  <img src="./img/ceb75301172038e75f5c43b328dd7febd7bedc7e.png" width="450">
+
+ - Fill in the installation path of the NDK.
+
+## Get source and dependencies
+
+ - Get Cesium sources:
+```bash
+git clone https://github.com/duniter/cesium.git
+cd cesium
+```
+
+ - Install dependencies (global then project's dependencies):
+```bash
+npm install -g gulp bower@1.8.0 cordova@6.5.0 ionic@1.7.16
+npm install
+```
+
+ - Install Cordova plugins:
+```
+ionic state restore
+```
+
+ - This should create a new directory `platforms/android`
+
+> To remind: check that your command line is configured:
+> - You must place yourself in the directory of the application: `cd cesium`
+> - and be configured for NodeJs v5: `nvm use 5` (please chek using the command `node --version`)
+
+## Android configuration
+
+Android need some configuration, to be able to build Cesium.
+
+ - First, create a properties file `local.properties` in the directory `platforms/android`:
+ 
+```properties
+# Path to your Android SDK installation
+sdk.dir=/opt/android-sdk
+
+# Path to your Android NDK installation
+ndk.dir=/opt/android-ndk
+```
+
+ - In order to sign your Android builds, create a file `release-signing.properties` :
+```properties
+storeFile=Cesium.keystore
+keyAlias=Cesium
+storePassword=YourStorePassword
+keyPassword=YourKeyPassword
+```
+ 
+ > Make sure to put your keystore file in the same path. 
+ > To generate a new keystore file, use:
+ > `keytool -genkey -v -keystore Cesium.keystore -alias Cesium -keyalg RSA -keysize 2048 -validity 10000`
+
+
+## Generate APK file
+
+ - Run the Android Build, to the signed APK file:
+```
+ionic build android --release
+```
+
+ - To get unsigned (debug) APK, run:
+```
+ionic build android
+```
+
+Generated APK files should be in directory `platforms/android/build/outputs/apk`
+
+### Troubleshooting
+
+#### "Cannot run program (...)/aapt"
+
+If you get this error:
+
+  ```
+  Cannot run program "/opt/android-sdk/build-tools/21.1.2/aapt": error=2, No such file or folder
+  ```
+
+Install two additional compatibility libraries (workaround found [this post](http://stackoverflow.com/questions/22701405/aapt-ioexception-error-2-no-such-file-or-directory-why-cant-i-build-my-grad)):                            
+```bash
+sudo apt-get install lib32stdc++6 lib32z1
+```