Gradle

I've converted the ExtrabiomesXL build scripts to be compliant with the new Gradle system Forge has introduced recently. In doing this I did need to move a lot of directories around, but all in all it is an improvement, and it looks cleaner to me.

It's a lot easier to use, for there's no need to install external build systems (especially since Ant is a pain to setup on Windows) as Forge comes with a built-in Gradle wrapper.

Doing this as a branch pull request so I can be sure everyone's on board with these changes and that there's no hidden bugs.

I'll also update the README.md when I have the time.