Album GetThumbnail and GetAlbumArt stubs

Added missing Album.GetThumbnail and GetAlbumArt to platforms where it isn't implemented (throws NotImplementedException).

The diff is ugly because I had to shuffle some code:
```C#
#if WINDOWS_PHONE || WINDOWS_STOREAPP
// Returns Stream (like XNA)
#elif IOS
// Returns UIImage
#elif ANDROID
// Returns Bitmap
#endif
```
to

```C#
#if IOS
// Returns UIImage
#elif ANDROID
// Returns Bitmap
#else
// Returns Stream (like XNA)
#endif
```