Add PUT/DELETE/HEAD methods (finally).

This commit renames certain POST centric variable and configuration
naming to the correct HTTP body stuff.

API changes include http_postbody_text() and http_postbody_bytes() to
have become http_body_text() and http_body_bytes().

The developer is still responsible for validating the method their
page handler is called with. Hopefully this becomes a configuration
option soon enough.