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 reflection. However WP8 is no-op so that works perfectly.

This PR also adds conditional compilation for WP, thus paving the way for a source package.