Usage statistics report

Implementation of the story from Trello: https://trello.com/c/9VAjy4le/61-usage-statistics
If it's good, I'll send a PR to `bundler-api` implementing the endpoint.
I think that we can use Net::HTTP::Persistent to re-use the connection to api.rubygems.org that we are already using for Rubygems data. This will create a new connection. Check fetcher.rb for an example of using the persistent connection!