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.
meta/stats: fix typos and add a test for non promulgated charms.
Make the use of prefix more explicit when aggregating download counts.
Improve the id/meta/stats endpoint response structure.
Merge pull request #241 from frankban/improve-meta-stats

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.