Implement GraphicsDevice.Adapter

This adds the Adapter property to the GraphicsDevice.

MSDN Documentation: http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.graphics.graphicsdevice.adapter.aspx

This changes the code a bit in a couple places, but the behavior should be the same. MonoGame currently assumes that we will always use the DefaultAdapter, but this may not be the case later on.