Ahoy integration

Adds tracking visits & events using Ahoy gem & Ahoy.js library.

For visit tracking, the MaxMind database is used (so the tracking is consistent with trade downloads). For that purpose a singleton class was added (lib/sapi/geoip.rb), which is now also integrated with trade downloads tracking.

Currently tracked events: accessing Species+ Search and Taxon Concept details. Event tracking from js is not enabled, as it was too noisy and possibly requires more thought.

There are pages in the admin panel to view visits and events.

Requires 'rake db:migrate`