Fix crashes when there's no albumArtist
Add datamodel.
Add Rdio data model.
Don't display cloud-stored media items.
Preliminary rdio implementation.

Not yet actually storing tracks to coredata.
Progress on data model.
List artists from Rdio works!

It's out of order, but it's there!
Add persistence to update state so updates can be resumed.

Also rename the CoreData models for Rdio to include the class prefix and
Rdio.
Update readme with current progress.