Go stateless.

The new semantics are as follows:
* Reap all containers that exited more than $GRACE_PERIOD_SECONDS ago (1h by default).
* Reap all images that don't belong to any retained container.

This is a stateless operation, so we don't have to rely on the /var/lib directory being populated. It's still there for internal use, but I'd rather replace that in a separate pull request.

The package now depends on python because doing date arithmetic in pure bash that works on both OSX and Linux was cumbersome. In related news, docker-gc now works on OSX.