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.
Hmm, that is a very good point. I was coming at it from the point of view of having no CSS dependencies - so you could get a "decent" looking nav out of the box. However, realistically the nav would be customized, which would mean each implementation would define a `pager-nav-divider` class that could set padding or content or whatever.

Or, are you suggesting removing the entire `<span class="pager-nav-divider">`, letting the implementer set content using something like `:after` on the `pager-prev` button?