Ability to retrieve page handler class

[benwerd]

Need to patch ToroPHP to be able to return an instance of the page handler class that would handle a given URL, so that we can choose an alternative method to call.

eg, for webmentions, need to get a page handler object as $pagehandler and call a function of the form $pagehandler->webmention($source).

[benwerd]

This has now been added, and is in use in the webmention endpoint.