PlaylistView: remove trackTableView reloadData from insertTracks:

- This is done automatically by receivedPlaylistSavedNotification, so
  it is redundant
MusicController: change FIFOBuffer refilling strategy

- Make FIFOBuffer sizes all be in seconds (i.e. bytes per sample times
  sample rate), so we have to make a new one per song (as size may vary)
- Start refilling it when it drops below 12 seconds
  - This is the main improvement of this commit as it decreases CPU
    usage fairly significantly.
  - Old way did a dispatch_async() on every playProc() call, which was
    probably slow due to GCD context switching. Now we only call it if
    we are ready to start refilling.
  - This bunches up the decoding into ~3-4 second chunks
- Stop refilling when it reaches 15 seconds
- Total size is 20 seconds just in case the decoder returns a huge chunk
  of data (i.e. no more than 5 seconds of data)
Merge branch 'master' of github.com:mileswu/dokibox