http: add NewProxyClientConn
http: add NewProxyClientConn

This just returns a ClientConn suitable for writing
proxy requests.  To be used in Transport.

R=rsc, petar-m
CC=golang-dev
http://codereview.appspot.com/4290052
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go