Added support for PCL - Profile 259

This includes all platforms that support .NET 4.5, except Silverlight 5

Most of the code is intact, supporting all the features that were in the iOS, Android, WP and most of NET45.
The only thing that cannot be supported is the cryptography, due to this being removed entirely.

I ran the unit tests on the actual PCL, and all passed. However, this did not test the OAuth integration, which may not work.

My OAuth knowledge is pretty thin, but I am assuming that the bits are not required to be encrypted, but should be for any sane developer. However, this library should work fine for apps that don't require OAuth, and if it is needed, the platform specific one can be swapped in (build against the PCL, but the app uses the platform version). Swapping will work as the API is exactly the same. This is going to be great to be able to use this lib in say Xamarin.Forms, and not require the platform-specific version.

Please let me know if things need changing or whatnot.