This commit adds a private `_slice` method to the Postpone class to
convert iterable objects into arrays. This method performs the conversion
by trying to call `Array.prototype.slice` on the provided object and then
defaulting to a manual iteration if the first method fails.

This commit abstracts some methods inside of Postpone so that the
execution of the methods is agnostic to their implementation and they
can be implemented to support non-standards-compliant browsers.

Added support for <object>

This commit accounts for an edge case where Postpone would not
correctly load elemnts 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.

This commit optimizes the internal `getElements` method, which Postpone
uses to locate and register all elements on the page who's resources
should be managed. This code change takes advantage of the fact that if
an element is already being managed by Postpone and the postponed
elements on the page have not changed, then the index of that given
element in the array of postponed elements should already be known. This
allows for the elimination of the `Array.prototype.indexOf` method,
effectively changing the line of code from O(n) to O(1).

This commit corrects an issue where the `start` method was not rebinding
events after these event listeners after these events were removed using
`stop`.