Enable Scroller only after Page Transition has Finished

Some page type do not animate in the scroller. So mouse wheel events
are triggered too early causing the text to already scroll even though
the page transition animation has not finished yet.
Add hidden_by_overlay CSS Class

* Hide page contents while overview is visible.
* Fix naming conflict with `hidden` class used to
  move elements off screen for accessibility.
* Set class on `section.page` not on `.content` to
  allow hiding text outside of content.
Merge pull request #183 from tf/hidden_by_overlay

Add hidden_by_overlay CSS Class
Merge pull request #184 from tf/scroll-back-bottom

Enable Scroller only after Page Transition has Finished