Reimplement pjax using pyquery

This is waaaaaaay simpler, and probably not too unperformant though I
didn't check.
Back out PJAX

Now that we've spiked out PJAX I'm backing it out of this ticket. I've
found what I think is a pretty good solution, which I've documented on
https://github.com/gittip/aspen-python/issues/206. However, it's going
to take a little more work than belongs on this ticket, because we're
going to have to manipulate nav in response to PJAX and not just
content. Also, it'd be great to use PJAX generally on the site. Let's
tackle that as a separate project.