Add stick at top when scrolling JavaScript

This lets you have elements that scroll with the page as you scroll.
This has been ported from the whitehall codebase but the tests updated
to use Jasmine rather than jUnit.
Trigger pageSizeChanged on show and hide links

So that the sticky scripts work in the correct place fire page size
changed on showing and hiding the primary link code.
Merge pull request #94 from alphagov/sticky

Add stick at top when scrolling JavaScript