Changed behavior of storing Spotify artwork

This fixes #7.  Previously Snip would store the artwork based on track
ID and not album ID.  This now only saves the artwork once for each
album.
Fixes issues while downloading Spotify artwork

This hopefully fixes #6.

* Added a WebException catch that will just save a blank image if an
error occurs.
* Increased the timeout from 5 seconds to 10 seconds.
* Changed the artwork download to be asynchronous.