PCL Project for Framework 4.5 supporting Xamarin

Created a portable project file which targets Framework 4.5, Silverlight 5, Windows Phone 8, as well as the latest Xamarin for Android and iOS.  This project is able to be referenced by the latest Xamarin projects.

The PCL 40 project, while still usable, is unable to be used as a project reference in the latest Xamarin for Android/iOS.