Add SBT configuration

Gradle scala plugin suffers from
http://issues.gradle.org/browse/GRADLE-3023
which prevents it working on Java 8, so on to SBT for our Scala support!
Update to libgdx 1.1.0
Update README with sbt info
Add SBT Assembly plugin
Make assembled jar executable directly
Java -> bad Scala

Convert Java code to Scala code.. but it's not very scala-y yet.
Remove Gradle build

Doesn't work anymore now that the project is scala-based