Android: support for "FragmentActivity" and "Fragment"

Hi Everyone,

I needed to use the Android Fragments with binding support into MvvmCross. But the current status of MvvmCross supports only the "Activity" class.

The following additions adds two user classes MvxFragmentActivity and MvxFragment followed by other Core classes to support their mechanism into the Framework.

The classes I added supports "FragmentActivity" and "Fragment" Android classes and are following the same mechanisms and structure of the ones supporting "Activity". The changes are mainly in the projects:
- Cirrious.MvvmCross.Droid
- Cirrious.CrossCore.Droid
I hope this additions brings an additional value to MvvmCross as it allows the Framework to handle bleeding edge Android APIs.

Thanks you very much