Include more info in the meta/stats endpoint.

The meta/stats endpoint returns aggregated downloads
count for the last day, week and month.
It also includes both the stats for the specific
entity revisions and the stats for all revisions of the
requested entity.
Can you add a test for the same charm with different names. /trusty/mongodb and ~charmers/trusty/mongodb. I'm sure I read that these need to match somewhere.