Fix constraint failures on song deletion

Deleting songs from the database is rare, but I sometimes do it when
troubleshooting problems in the playlist.
Only play songs that exist
Merge pull request #38 from williamjacksn/master

Fix constraint failures on song deletion
Re-read tags after mp3gain
Add total amount to tip jar page
Add 2 displayed fields to tip jar and keys
R4: Speed up album rendering
R4: Use tables for playlist now
R4: Improve playlist performance
R4: Load playlists before anything else
R4: Fix ratings in album list
R4: Fix playlist again
R4: More attempts at speeding up playlist
R4: Work around offsetHeight calls in playlist
R4: More playlist speed attempts
Merge remote-tracking branch 'upstream/master'