Create a Google Analytics event tracker

* Call GOVUK.track with action, label and value
Auto-track events based on markup

* Create a helper module to pass significant events to GA using markup
Track use of bulk edit and bulk add tools

* Track multiple selections in modal window
* Track when mappings are saved, with the type of action performed and
the number of mappings updated
* update-single, bulk-edit-archive, bulk-edit-redirect, bulk-tag,
bulk-add-archive-overwrite-existing, bulk-add-archive-ignore-existing,
etc.
Merge pull request #211 from alphagov/ga-events

Track JavaScript interactions in Google Analytics