Initial attempt at pageable incident list

* Upgrade Backbone to 1.1.x releases
* Convert collections to using PageableCollection
* Configure PageableCollection to (mostly) interact with PagerDuty's
  paging model based on offsets vs. pages

At this point I've proven the paging can work, by visualizing only the
100-200th most recent incidents, but this still needs work to actually
load more than one page of results and render that.