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.
I didn't know about the eq() function, useful.

About the replace, that's not for trimming extra whitespace (although that would be a good idea anyway). It's there to convert different types of whitespace (eg nbsp) to standard spaces to ensure non-breaking spaces don't interfere with the comparison.

```
$('<p> </p>').text().charCodeAt(0);   ===>  32
$('<p>&nbsp;</p>').text().charCodeAt(0);   ===> 160
```