Fix bug in `filter`

Selectors passed to the `filter` method should only be applied to the
top-level elements in the selection. In other words, descendent elements
should *not* be considered when filtering by a selector string.

From [the jQuery documentation on `$.fn.filter`](http://api.jquery.com/filter/):

> **Description**: Reduce the set of matched elements to those that
> match the selector or pass the function's test.

This should resolve issue #158.