Added ScalaTest to library dependencies for tests
Added all-platforms project aggregate

Aggregate project allows to compile or test code from all platforms.
It allows to run all */src/test/scala/* with single "test" sbt command,
yet still it allows to run separate test suites with eg. "common/test"
Added example ScalaTest spec file

It is very good practice to test code. To follow famous android-app.g8
template, I've added simple placeholder for testing. But should we unit
test games? Definiately! Lots of bugs can be avoided that way. Of course
we cannot test visuals that way, but everything related to mechanics,
game world, stuff like that - it is extremely useful. Lets add simple
template for people to get started with (if they don't want it, they can
easily get rid of it by just removing test directory).
Added integration tests for Android

Added integration tests for Android, modeled after jberkel/android-app.g8

To run, install app in emulator, then issue:
> project android-tests
> android:install-emulator
> android:test-emulator
Added info about tests to README
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.