Rename request/response classes to better match Reel

This factors the classes used by requests and responses within the namespace of
the Request and Response classes. The names are changed to better match Reel:

* HTTP::RequestStream  => HTTP::Request::Writer
* HTTP::ResponseParser => HTTP::Response::Parser
* HTTP::ResponseBody   => HTTP::Response::Body

This provides a more incremental refactor towards what I was trying to accomplish in #72. It also represents the last of the major API changes I actually want to make, so perhaps after this is merged we can cut a release and save #72 for the next version.