Changed date search logic.
Changed date search logic.

- We cache the most recent acquire/move dates and only search if there's
  at least one track in this directory that's after the date we're looking
  for.

- We return a list of tracks that are in m3u playlist format. Not sorted.
- Instead of giving a number of days and requiring that we keep re-calculating
  a timestamp, we pass in the timestamp that we want to look relative to.

bgporter/mp3utilities