Cleanup some doc strings

Fixed a typo in SecretBox docstring

Filled out the readme

Enable running tests from setup.py

Unfortunately, this only works if cffi is already installed

Add nacl.secret.SecretBox - Secret Key Encryption for PyNaCl

Fixed warning in docs creation

* Moved image to the static_path

* Adds nacl.secret.SecretBox which represents a secret key.
* nacl.secret.SecretBox().encrypt can be used to encrypt a message
* nacl.secret.Secretbox().decrypt can be used to decrypt a message

Add an Encoding system instead of adhoc uses of binascii

* This eliminates the global state from the encoding system.

* Adds an Encoding system that supports raw, hex, base16, base32,
  and base 64.
* Adds an encoding=ENCODING parameter to methods to control what
  encoding data is returned/passed in.
* encoding parmater supports passing in a custom encoder object
* Encoding system supports registering of new encoders through
  nacl.encoder.register.

Update libsodium to 0.3

Fixed ignore file

apostrophe & capitalization in docs/signing.rst

Digital signatures