use /var/lib instead of /var/run

Derp. /var/run is cleared on reboot, breaking docker-gc.
Find containers to reap by inspecting their exit timestamp.
Dont rely on filesystem state.
Dont use "-r" option for xargs because it doesn't exist on OSX.
Depend on python.
Update documentation.
Merge pull request #1 from mbruggmann/exited-timestamp

Awesome!