Add clean command and ps status filtering.

Add "docker clean" which removes all exited containers. This is especially
useful for testing and systems which spin up and shut down containers often.

docker clean finds all exited containers and removes them from the daemon.

Also adds a new filter facility to the "docker ps" command:
`--filter="status=<int>"` where the codes are

0: unknown
1: restarting
2: running
3: paused
4: exited

The /containers REST resource also returns an additional field per
container ("StatusCode") which contains this code for each container.

Signed-off-by: Henning Schmiedehausen <henning@groupon.com>