Implement `$.fn.end`

From the jQuery API documentation[1]:

> **Description**: End the most recent filtering operation in the
> current chain and return the set of matched elements to its previous
> state.
>
> Most of jQuery's DOM traversal methods operate on a jQuery object
> instance and produce a new one, matching a different set of DOM
> elements. When this happens, it is as if the new set of elements is
> pushed onto a stack that is maintained inside the object. Each
> successive filtering method pushes a new element set onto the stack.
> If we need an older element set, we can use end() to pop the sets back
> off of the stack.

[1] http://api.jquery.com/end/

This should resolve issue #199