MediaObjects need a way to get (and store) metadata

At the moment there's no way to get any metadata about the file stored in a MediaObject. It would be useful to be able to do this, since we could store, say, the EXIF data about an image and display it to the user later.

One way that we could do this would be to add a MediaObjectMetadata Model to frabjous.media.models. This would have a many-to-one relationship with a MediaObject. MediaObjectMetadata could just be a key => value pair store.

Alternatively, we could serialize the metadata as a JSON struct, which is the slightly nicer solution, since it would keep the metadata with the original MediaObject.