Add command `weave ps`

This shows all running containers attached to weave on the local host, with their name, image, hostname and IPv4 addresses on the weave network.  #138 relates.
This comment is no longer correct, after recent changes on master, in particular https://github.com/zettio/weave/commit/ded7d3a9913e7330d235948b2f3a6aa1fb6cc9bd

I suggest ditching the comment altogether.