FNA Compatibility

To make getting up and running with FNA easier there are separate csproj files (*.FNA.csproj). Note that you still have to install the required FNA native libs per the FNA documentation. The MonoGameCompat class is included as well and consists of a few extension methods that implmement some commonly used method that are in MonoGame but not in FNA.

See the "Using Nez with FNA" section of the README.md for setup details.