GetBackBufferData implementation for OpenGL (Others are just stubs currently).

Docs:
http://msdn.microsoft.com/en-us/library/ff434115.aspx
http://stackoverflow.com/questions/1352864/how-to-get-uiimage-from-eaglview/1945733

refs #1885
DirectX implementation of PlatformGetBackBufferData
Tidy up OpenGL implementation of PlatformGetBackBufferData to match and comment the public function
Fix GL.ReadPixels call for IOS
More comments on pixel formats by platform
Support non byte[] for GL GetBackBufferData (Thanks @thefiddler)
GetBackBufferData Windows Phone support
Fix build on MonoMac
Merge branch 'getbackbufferdata' of https://github.com/danzel/MonoGame into pr2382

Conflicts:
	MonoGame.Framework/Graphics/GraphicsDevice.OpenGL.cs
	MonoGame.Framework/Graphics/GraphicsDevice.cs