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.
Oh, yeah, it actually only returns 200s... The `wp-request` module actually checks to make sure that the response was in the 200s, otherwise it returns an error object. It looks like the value of `body` in the case of an error is `{ status: 'error', error: 'Not found.' }`... I'm not a big fan of testing against the string `Not found.` because that seems like a human-readable string that might be apt to change, even depending on the locale... Hmm. But at the same time, if we don't test against it, then you're right, we could be masking some other kind of error as a 404!