Ampache album cover support

Scanner assumes that user have "cover.jpg" in album directory (following convention: Artist/Album/cover.jpg)

Screenshot: 
![screenshot 04 02pm lis 30 2014](https://cloud.githubusercontent.com/assets/783794/5238107/4e989954-78ae-11e4-9ea9-2b8699fa829d.png)

This part is done as background job and can be found at https://github.com/owncloud/music/blob/master/db/albummapper.php#L261 where 'cover', 'albumart', 'front' and 'folder' are possible cover names ;)