Added support for AntiForgeryTokens on .NET 4

The main project now targets .NET 4, so the separate .NET 4 project has been removed. I didn't see a .nuspec, so you'll probably need to tweak it to just include a single net40 lib (which will work on .NET 4.5).

I've tested the changes on MVC 3, 4, and 5.1.2 apps running on a machine with the latest .NET 4.5 installed.