ImageParser deal with BitmapAsset embeds

[richardolsson]

When embedding images, they will by default be mx.core.BitmapAsset instances, meaning that a common mistake will be for users to have ImageParser try to parse data that is not actually a byte array, but instances of said class. Deal with that; either throw an informative error, or instantiate the class and return it's BitmapData as an asset directly (instead of using Loader.loadBytes() to load the bitmap.)