Move request line and status line out of RawHeaders.

The request line is now standalone because it depends
on the proxy (for full URL or not) and the connection
(for HTTP version). I may change this later and put
this info in the request.

The status line now moves into Response. I have a
helper class StatusLine that does the parsing. I may
later want to hide this or make it public; I'm not
quite sure yet!

The net result is that RawHeaders is getting dumber
and used in fewer places, which is good news. This
change also shows nice simplifications to the
authenticator (which needs fewer args) and to the
tunnel builder.

Also fold together HttpsEngine and HttpEngine; the
HTTPS subclass wasn't pulling its weight.
Merge pull request #382 from square/jwilson_1230_requestline

Move request line and status line out of RawHeaders.