Create a Bitmap abstraction for RxUI

This PR makes it so that ViewModels can handle bitmap loading / caching.

Here's the idea:

```cs
//
// This is in the ViewModel - we can share this on every platform without having
// to put our image loading code in the view
//

LoadTheImage = new ReactiveCommand(
  this.WhenAny(x => x.CurrentFilePath, x => File.Exists(x));

var loadedImages = LoadTheImage.RegisterAsync(_ => 
  BitmapLoader.Current.Load(File.OpenRead(CurrentFilePath)));
loadedImages.ToProperty(this, x => x.LoadedImage, ref _LoadedImage);

//
// Meanwhile in another part of town, the View....
//
this.OneWayBind(ViewModel, x => x.LoadedImage, x => x.ImageControl);
```

## TODO:

- [x] Finish up Android / iOS
- [ ] Implement it in WinRT + WPF + WP8
- [ ] Binding converters to native types