Initial configuration of the GHUnit testing framework

This includes the initial configuration of the GHUnit testing framework I mentioned in https://github.com/wikimedia/Commons-iOS/issues/23. I added a separate target in the project. 

The tests run in the simulator and on an armv7 device but armv7s (iPhone 5) is not yet supported. The only device I tested is the iPad 3.

I've also added the login test that I was working on before.