Work around unicode and bad mp3s.

Some MP3s can't be scanned, and picard throws an exception, so we check
for that.

We also coerse str strings into unicode strings using SQLAlchemy black
magic. It isn't pretty, but it's what the SQLAlchemy documentation
instructed us to do, alas.

Reported-by: James Pic <jamespic@gmail.com>
Properly support WAV files.