Updated readme
[Client] Moved GUI XML files to resources
[Engine] Properly shutdown loading executor once loading is done
[Build] Improved build files

Application now can start using gradle
Removed old useless code
Removed a load of old and totally useless files
[Build] Trying multiple methods to access git
[Build] Cleanup build script
[Download] First working version of the downloader

The downloader has yet no GUI, but its able to download and launch the
client.
[Build] Improved build scripts (maven publish target)

Its now possible to set by command line where the maven repository that
stores the artifacts is located.
[Nifty-GUI] Fixed broken compiling

The compiling of the client was broken due a change of the name of a
function in the Nifty-GUI. This issue is resolved.