Implements downloads counts (for dataset resources) and fixes an issue with 'All' records.

- Fetches the data for downloads (either direct or cached) from when a user clicked on a
  Download button.  We lookup the resource for this url (based on .url or .cache_url) and
  then associate the download count with the package it belongs to.
- Fixes a bug (#211) where the All records are deleted for every url, even if that url was
  not fetched (and therefore won't get a new All record).