Add logic to update <title> on navigation, to pass our breaking test

Parse the responseText with a regular expression to identify a <title> element,
then set the document.title to the matched string.
Update the URL in browser history when loading new images

This prevents users from experiencing a disconnect between the image shown/title displayed, and the URL. If their browser supports it, we swap out the URL at the same time we swap out the title of the page.