This PR makes it so that ViewModels can handle bitmap loading / caching.

Here's the idea:

//
// This is in the ViewModel - we can share this on every platform without having
// to put our image loading code in the view
//

LoadedImage = await BitmapLoader.Current.Load(File.OpenRead(CurrentFilePath)));

//
// Meanwhile in another part of town, the View....
//
LoadImage.ImageSource = ViewModel.LoadedImage.ToNative();

TODO:

• Finish up Android / iOS
• Implement it in WinRT
• Implement it in WPF
• Implement it in WP8
• Figure out how to register the native loaders with the platform-independent bit

Yes, Microsoft invented three separate ways to load bitmaps. No, I'm serious.