Database reliability fixes.

1) Match the column list to the Track.__init__ parameter list (manually, not
   automatically, though that would perhaps be worth doing)
2) Use parameterized queries rather than interpolation
3) Slightly dehackify the status check in get_many_mp3()
4) Correct a docstring to match behaviour
Merge pull request #79 from Rosuav/db_reliability

Database reliability fixes.