PCL Support 

This addresses both #23 and #30. 

It adds a PCL project targetting WP 8.1, .NET 4.0, and Xamarin IOS/Android with a stub Purify method. It uses "The Trick" (@paulcbetts) or as I like to now call it "Bait and Switch" technique to allow overriding with the Platform specific implementation as WP8 does not support the same reflection APIs.  We don't need to reflect anyway for WP8.1 because PUrify will be no-op anyway, similar to 4.5.

This PR also adds conditional compilation for WP, thus paving the way for a source package.