It's time to beef up the queue.

• Don't queue the same file twice.
• If a queued file is not in the push, don't push it.
• Add a command to gc the queue.
• Look for other Git Media objects in the queue to push.

These features are too much for the queuedir package. It'd be awesome to find a good Go based k/v store that'll work cross compiled to every platform.

Fixes #82 and #99. /cc #102 (comment)