Feature/es8

Turns on the exclusive usage of NodeJS 8 for Duniter.

NwJS has been upgraded to 0.24.4 for this purpose, and Duniter no more allows the usage of NodeJS 6.

I've already tested these 3 artifacts:

• duniter-desktop for Linux (.tar.gz, .deb)
• duniter-desktop for Windows (.exe)
• duniter-server for ARM

Everything works fine, I could synchronize and join the network.

I've also introduced a new building feature to create testing artifacts:

./release/scripts/build.sh make deb

This command will build Linux (.tar.gz, .deb) locally with YEARMONTHDAY.HOURMINUTE.SECONDS version name. One can also use:

./release/scripts/build.sh make win
./release/scripts/build.sh make arm

Or the generic version which builds all 3 at once:

./release/new_prerelease.sh

If no tag is given to new_prerelease.sh, then it makes a local build. No artifact will be uploaded to GitHub.

You can see I've added a 0.24.4_common.gypi file for both Linux and Windows builds. That a fix for nw-gyp, which is bugged and without this file (which is the good one), we could not build NodeJS Addons properly and Duniter could not run. Once we will migrate to NodeJS 10, we will be able to remove these 2 files.