Multithreaded Cover Management

Try to manage several threads to retrieve covers. This way we can enable it in the playlist view, even with big playlists.
I think there are plenty of room for improvements but it works well on my device (Nexus 7).
The web fetching from the playlist view is only done if the  enableAlbumArtLibrary property is enabled (a little hack to test it safely).

The main thing is a blocking queue where cover requests are added. Then several threads process the requests : 
- 2 for web requests
- 1 for priority requests (playing now)
- 1 for the cache and bitmap making

I guess we may change these settings (too many thread ?)  if needed afte som tests.
I got some problems with bitmap recycling but it seems to work fine now

I clearly need some feedback to know how it performs on different hardwares.

Let me know what you think.