cross-compile multiple Scala versions and upgrade to SBT 0.12.1

Since scala does not maintain binary compatibly between versions, organizations tend to have to move all of there code at the same time. It would thus be very helpful if we could cross build multiple scala versions.

http://code.google.com/p/simple-build-tool/wiki/CrossBuild

Unclear if this would require KAFKA-134 or just work.