Content seen tracking

This PR adds analytics tracking measuring how far a user scrolls down a given page.

Pivotal: https://www.pivotaltracker.com/story/show/69606914

Tracking can be added to pages by path, sending events when the user either scrolls to a certain percentage or scrolls to show a given heading.

This also includes a fix for GOVUK.sendToAnalytics() which we (I...) seem to have broken.
Yes - scrollTracker is the instance, ScrollTracker is the constructor.