Account for re-rendering

This release accounts for an edge case where Postpone would not correctly load elements when they had scrolled into view. Specifically, it takes care of the case when a set of already postponed elements are completely re-rendered by an application. Re-rendering a page with the same content may result in completely equivalent markup to the markup that existed before, but from a browser's point of view, the DOM nodes represented by those sets of markup are not identical. Postpone was treating these nodes as identical, so if your Backbone app re-rendered a bunch of still un-loaded images, Postpone would be unable to load them.