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.
Hm, I do agree that `.aria-hidden` may not be the best class name. Perhaps `.completely-hidden`? `.presentationally-hidden`?

I'm unsure on the `hidden` attribute. The docs have this bit which gives me pause:

> For example, it is incorrect to use hidden to hide panels in a tabbed dialog, because the tabbed interface is merely a kind of overflow presentation