Add Calabash along with some minimal test coverage for Android and iOS

Requirements
-------------------
  1. Install Calabash via [http://docs.xamarin.com/guides/testcloud/calabash/installation/](http://docs.xamarin.com/guides/testcloud/calabash/installation/)

Running iOS Tests
------------------------
  1. Build XamarinStore.iOS
  2. In the command line, from within the ```XamarinStore.iOS``` directory run ```cucumber```

Running Android Tests
------------------------------
  1. Build XamarinStore.Droid in either ```Debug``` or ```Release``` mode
  2. Start an Android emulator
  3. In the command line, from within the ```XamarinStore.Droid``` directory run ```calabash-android run ./bin/[Debug|Release]/com.xamarin.XamStore-Signed.apk```

More Info
------------
See: [http://docs.xamarin.com/guides/testcloud/calabash/](http://docs.xamarin.com/guides/testcloud/calabash/)

One Last Note
-------------------
If this were not a contest ending tomorrow morning, I wouldn't consider this a finalized pull request. I'd likely do the following.
  1. Much more thorough testing
  2. Consider refactoring to an organizational approach closer to the one used in the TaskyPro example [here](https://github.com/xamarin/mobile-samples/tree/TaskyPro_Calabash/TaskyPro/features).
  3. Learn enough about Calabash/Cucumber for Android that I could implement the same exact tests on each platform.