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.
I wondered if this divider should be done with CSS only, rather than a pipe character. I realize you can set the divider character and provide custom CSS, but given how much power we have with CSS now, I'm not sure it's necessary to have the divider in markup at all.