Only send -didCompleteLoadWithSessionID if we got a media status response to a LOAD command
Handle media LOAD_FAILED responses
Store media information between media status updates

This ensures that we can always return a full dictionary of media
information rather than the partial updates that the Cast devices
supply.
Retrieve the currentTime property from media status updates
Use the default source ID for changing volume/mute status
Add simple container classes: OCMediaReceiver{Application,Status}
Track the receiver's status and report it to the delegate