Add scroll tracker

This adds a scroll tracker which can be configured to track when
a user scrolls to either a set percentage down the page or to
reveal a below-the-flod header.
Configure scroll tracker for initial trial
Repair GOVUK.sendToAnalytics

A previous commit scoping scripts inside closures broke GOVUK.sendToAnalytics
by scoping _gaq. Google analytics overwrites _gaq when it initializes so
having it scoped isn't good.