Adds support for draft HTTP error code '451: Unavailable For Legal Reaso...

I'm possibly jumping the gun a bit since the draft of this status is brand new, but I wanted to get this out there while I was thinking about it (also, RIP Ray Bradbury).

It's probably reasonable to hold off on merging this request until such time as the draft is accepted or until it gains a little traction, at which point the docstring could probably use an update as drafts aren't supposed to be cited for reference.

The draft can be perused at:  http://datatracker.ietf.org/doc/draft-tbray-http-legally-restricted-status/  -- this seems to be a decent place to watch it for updates.