Improve mozilla-pager.js. Bug 966702.

- Remove unused 'pager' JS bundle.
- Add docs pages for mozilla-pager (Bug 1007717).
- Add mozilla-pager.js demo page to styleguide (dev only).
- Add ARIA support for mozilla-pager.
- Add ability to create/destroy pagers.
- General performance/cleanup of mozilla-pager.js.
- Updated /firefox/desktop/tips/ and
  /firefox/channel/ to use newer pager features.
So I believe just adding `aria-hidden` is not yet quite enough, since some screen readers don't support it yet. 

When testing with VoiceOver on OSX for example, I can still read the content for each page even when its set to hidden. I would also apply 'display: none;' in addition here.