Add websocket functionality

[ptesavol]

Expose the WebSocket functionality of libdatachannel in node-datachannel

• Change the compilation options of libdatachannel to enable WebSocket functionality
• New Classes WebSocket and WebSocketServer
• Simple websockets.js test
• Typescript typings in which both DataChannel and WebSocket implement a common Channel interface so users can handle them in an uniform manner

ToDo:

• WebSocket implementation is, for most part, a cut-and-paste from DataChannel functionality, maybe we should extract the common functionality into a superclass?
• Add jest tests