HTTP::ResponseBody

This factors all of the concerns of processing the response body into a separate
HTTP::ResponseBody object. This is a *BREAKING* API change, but hopefully one
that results in both a better API and a cleaner, more maintainable codebase.

It also deletes the callback system, which is 1) unnecessary 2) getting in the
way of the refactor.

Redirects are presently broken and marked as pending.