Another take on HTTP mode

[utrack]

This PR adds communication over HTTP, so it would be possible to use existing server to serve gops traffic. All previous modes are preserved as-is; so both local and rawsocket-remote modes are still there.

I've refactored client code a bit so that both socket and HTTP modes would be possible to use; now there's a Client iface implemented by ClientTCP and ClientHTTP that is passed to every gops command's function instead of raw TCPAddr.

To use it run gops like this: gops pprof-cpu http://localhost:port/handler