Features/proper player

this PR adds the following:
 * metadata editing for songs
   * server side and client side
   * the client side is prototyped on the upload screen but can be easily integrated into another view
 * the ```owner_id``` is set on song upload
 * the ```/song```-resources check ownership of gridfs objects and songs now
 * ability to redownload covers 
   * useful if you upload a song with wrong metadata and want to get the correct cover later
 * a new sidebar with a proper looking player
 * a queue service that allows you to add, reorder, remove songs
 * drag and drop for this queue
 * a placeholder for a "save playlist" button
 * several smaller cosmetic fixes
 * i removed the ng-boilerplate branding and the google+ button library as well


this PR is not meant to be merged, since there aren't enough tests yet. 
**given that it is feature-complete though, you can use this branch for your own work. any future bugfixes that are likely to arise from testing are only minor and will not create error when merging**