Support Xamarin.Android and Xamarin.IOS in Stateless PCL

The PCL Profile (Profile2: portable-net4+sl4+netcore45+wp7) does not support Xamarin.Android or Xamarin.IOS. This changes the profile to  Profile136:portable-net4+sl5+netcore45+wp8+MonoAndroid1+MonoTouch1.

Profile 136 does not drop support for any platform that Profile 2 supported. However, it does increase the version required for Silverlight (from 4 to 5) and Windows Phone (from 7 to 8), because there is no PCL Profile that supports Xamarin as well as these old versions.