Remove the use of DDG.strip_href

We no longer need to use DDG.strip_href for making HTML safe for DDG.strip_html. DDG.strip_html does everything for us and no longer executes scripts or loads images.

In a more technical level, we're no longer using the solution described in http://stackoverflow.com/a/5002618 (which loads images and executes scripts). We're now using the same solution as the one implemented in https://github.com/epeli/underscore.string/blob/master/lib/underscore.string.js#L199