Implement 3xx Statuses

[sleepdeprecation]

The 3xx status codes are kinda tricky, and require extra headers. It might be useful to do real HTTP 3xx stuff, like 301 or 302.

Also, 304 / other caching stuff. See also #7 .

[sleepdeprecation]

Considering that we now have the ability to add arbitrary headers (see #16 / #23), this shouldn't be overly difficult, because they're all just extra headers...