Add FxOS anniversary page. Bug 1032279.

- Add GA to FxOS anniversary page. Bug 1046844.

- Add 'findPageById' to mozilla-pager.js.
- Add options param to mozilla-pager.js to allow for
  onCreate callback.
- Document 'currentPage' property of mozilla-pager.js.
- Move custom easing methods in mozilla-pager.js out of
  document.ready.
- Standardized indentation on firstrun.js (no code changes).
Is `role="presentation"` needed here? @icaaq ?