Fetch album cover in background thread/async to avoid blocking UI
Move cover fetching into the setter method of the LibraryViewAlbumCell

This is to prevent the fetchCoverAsync being called multiple times
Spinning progress indicator during album cover async load

NB: This causes weird flashing on the entire table view: it causes
all of them to flash white sometimes.
Merge branch 'master' of github.com:mileswu/dokibox