Added support for HTTP/S proxy.

Control using http_proxy, https_proxy, no_proxy environment variables.

Note: I'm aware of the commit 76aff601f2bfa084d67b39acbdfb8dde40a5feb2 but I didn't like it because of the lack of support for no_proxy, lack of support for http proxy, and the .request() url was relative rather than absolute so the request ended up failing anyway. I couldn't reproduce the TLS problem requiring force to TLSv1.