Added ScalaMeter

Added ScalaMeter, a microbenchmarking and performance regression testing
framework. Performance is very important for games, and thus it is good
habit to regularly check performance critical fragments of our games.
Fixed ScalaMeter on JDK6

ScalaMeter needs extra option on JDK6, because it uses some GC features
from JDK7: https://github.com/axel22/scalameter/issues/3
Because Android works best with JDK6, I emabled -preJDK7 option by default.