SBT integration

This SBT integration gets rid of SBT LWJGL Plugin. I chose to use a Build.scala, this is really cleaner and more flexible. I used some parts from the code of SBT LWJGL plugin anyway.

I updated the README as well because i developped a plugin which is simple enough to use : https://github.com/mvallerie/sbt-scage-plugin

I'll create a template ("archetype" in Maven world) when i'll get more free time.

Enjoy.