Refactor dependencies to allow PCL sharing between Windows & Xamarin.Forms

[Andy-Wilkinson]

Currently the Xamarin.Forms support is in a separate NuGet package (since there is the possibility that a Windows Phone project is based on either the core packages, or the Xamarin.Forms packages). Unfortunately this means that a shared PCL library cannot be consumed by both Windows and Xamarin.Forms projects.

The proposed changes are to transfer all portable code into the PCL (currently they are just stubs that are then replaced by the platform specific binaries). This can be distributed via an Okra.Core.Portable NuGet package which would be referenced by platform specific packages as required (which will then only contain the non-portable code).