Improve the process of updating the UI after rating a song.

* The main playback view and History drawer could show different UI states for the thumbs up/down buttons when the playing song was selected in the History drawer and then rated.
* The spinner is now always displayed when rating a song. It was not previously displayed when rating from the History drawer.
* Calling "thumbs up/down" from AppleScript now updates the UI.
* Fixed a bug with [Pandora -deleteRating] ignoring thumbsDown feedback entries.
Fix state of thumbs up/down buttons in History drawer when no item is selected.
Merge pull request #124 from dlh/rating-ui

Improve the process of updating the UI after rating a song