Added additional stat types

- Added 'capabilities.json' which is basically a reverse cumulative
  'installations.json' to assist plugin developers in choosing base
  versions. Assumptions break down with backported API, but close enough.
- Added 'PLUGIN.versions.json' which contains the most recent month's
  distribution of versions of a specific plugin to assist administrators
  in making update decisions.

Minor changes:
- Ensure installations.json is based on latest month only. When I ran
  the script (no access to the raw stats), the DB contained all months.
  Not sure this is actually needed.
- Changed call order to place the long running process at the end.