Support for Unit Testing (Desktop and Android) and Instrumentation Testing (Android)

Testing frameworks are very important for my style of working, and I believe everyone can benefit from some unit or instrumentation tests. I've used those features in jberkel/android-app.g8 and missed it from this template. Those 4 commits are a try to bring in slightly simplified version of those in.

The biggest change is probably adding "all-platforms" aggregated target. That way, we can compile or test all platforms at once by issuing single "test" sbt command. Instrumentation testing is little harder, but it uses commands from android-plugin so is quite standard. Used android-tests for it to underline, that it is android based.

If someone does not need tests, this does not change much - one can just ignore those then and use everything like it was. Library dependencies are added only when test is used (it has % test as qualifier).

I also provided one (pending) unit test in common, and one actual test in Instrumentation test as examples.