Ported MonoGame to use the new Xamarin iOS Unified API

This is a port of MonoGame to the new Xamarin Unified API on iOS (http://developer.xamarin.com/guides/cross-platform/macios/unified/). The Classic API is a dead end on iOS, as Apple requires all apps to include a 64 bit binary, starting February 1, 2015 https://developer.apple.com/news/?id=10202014a



Protobuild doesn't support the unified api yet, so you'll have to modify the project files (MonoGame.Framework.iOS.csproj and MonoGame.Framework.Net.iOS.csproj) for now by hand. Change the ProjectTypeGuid from
 ```6BC8ED88-2882-458C-8E55-DFD12B67127B``` to ```FEACFBD2-3405-455C-9665-78FE426C6842``` 
and replace the occurence of 
```<Import Project="$(MSBuildExtensionsPath)\Xamarin\iOS\Xamarin.MonoTouch.CSharp.targets" />``` or ```<Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />``` 
with
 ```<Import Project="$(MSBuildExtensionsPath)\Xamarin\iOS\Xamarin.iOS.CSharp.targets" />```

These changes need to be done on the ThirdParty/Lidgren.Network/Lidgren.Network.iOS.csproj project file too, and you need to change ```<Reference Include="monotouch" />``` to ```<Reference Include="Xamarin.iOS" />```

@dellis1972 is there a constant in xamarin.ios that matches the OpenGLESLibrary constant in MonoTouch? It would be smarter to use that here.