Add support for connecting to TCP sockets

[rgl]

Currently we can only connect to unix domain sockets, but its also useful to connect to TCP sockets.

To distinguish between the two, we should support something URLs, something like tcp://{ip}:{port} and unix://{path}.

To maintain backwards compatibility, it should be enough to assume its a unix domain socket when the user did no supply an URL (e.g. {scheme}:{rest}).

What do you think?