Add Client.listall method
Cache album artwork.

Play has to shell out to get album art for music. This makes play cache the
images so it doesn't have to shell out.

Album artwork is saved to play/public/art. The filename is generated by
creating a SHA hash of the Artist Name and Album Name and is a png image.

When the play app is started it creates a new thread to precache the artwork
for all the music the MPD library. When there is no album art or the art
hasn't been cached yet the art-placeholder.png is sent in place.
Merge pull request #199 from joeyw/cache-album-art

V3 Cache album artwork.
Only cache in distinct, out-of-band task
Cache in bootstrap
Easy queue- just use mpd's settings for now

(We can add smart playlisting on presence later.)
This is important enough to do all the time
Actually add songs
Test without a user
Fix song link
Limit most finds to 50 songs
Set an height
Add an album link, too
WE NEED MORE FILE DESCRIPTORS
Simple duration
Don't initialize all objects
Install mpc too
Better status
Report on music server status
Monitor the queue process too