Very simple implementation of 404 page.

This is a super simple implementation of a 404 page that fixes #32. In the future, if we want, we could expand it to work more like #33, which puts the 404 page within the single-page app.
Hmm fair enough. At the very least, I think this demands some tests, but I'd also really like to get this particular change out the door very soon since I suspect there are lots of 404s, and at the very least I want to be [recording them](http://www.bluefountainmedia.com/blog/track-404-errors-in-google-analytics/) through google analytics (which I'll add in a separate commit). Do you think it's reasonable for me to file a separate bug about refactoring the error handling, but commit this now?