Create a new WebSocket client interface

[erikedin]

I feel like I didn't make a distinction between a WebSocket client and a WebSocket connection when I created the public interface to the package.

It is explicitly mentioned in the specification that a client can maintain different connections. Today I don't make any such distinction between client and connection.

[erikedin]

Added to the "Requirements coverage" project, because some requirements are unimplemented because of this.