Added HttpListener-based websocket channels

[yallie]

Also added a bunch of interoperation tests:

• System.Net.Websockets client → HttpListener server
• WebSocketSharp client → WebSocketSharp server
• System.Net.Websockets client → WebSocketSharp server
• WebSocketSharp client → HttpListener server

Moved WebSocketSharp-based channels to their own namespace.
It's now possible to extract these channels to a separate project to get rid of WebSocketSharp dependency.

See also these discussions:

• System.Net.Websockets #76
• Sometimes RemotingSession closed due to loss of connection in WatsonTCP #75