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.
I have been unable to find this discussion on the issues page online,
and am only seeing it in my inbox.  I had hoped to see it in context,
but that is proving difficult.  

For the benefit of those of us not testing every new commit, could
someone please post some sample output from this new ps sub command.  

Personally I want to see not a crytpic hex code container ID but the
image name and the host name and the ip on the weave bridge, and
perhaps some report of its connectivity.  I like the idea of a -v
(erbose) switch as a helpful compromise with those who would prefer a
less is more approach.  I could also imagine a -f(ormat) approach, 
which might support multiple or even custom options.  

I have already added a helper script to my local installation which
crudely provides some of this, but would appreciate having this built
into the weave script.  

-- Hugh

-- 
Hugh Esco <hesco@yourmessagedelivered.com>