Discovering meters from the API

Implement /meters to make discovery "nicer" from the client

The point of this api is to make discovery (esp. from a casual user)
easier. So you don't really want to dump all the raw samples out
just to see what is there. So instead "ceilometer meter-list"
will GET /v1/meters (or /{proj|user|source}/{id}/meters) and
this will just return a description (name, type, resource, user, etc)
of the available meters, not each sample point. After this you will probably
go and look at the samples that you are actually interested in.

It is a kind of dynamic version of doc/source/measurements.rst