Typescript + Under the hood improvements

This release contains some under the hood improvements to better align itself with the spec. It also includes a typescript definition file, and bug fixes.

API breaking change:

const mockServer = new Server('ws://localhost:8080');
mockServer.on('connection', socket => {
  // the first argument is now a socket reference instead of the server instance.
  // this now allows you to send directly to the individual socket that connected.
  socket.send('test message 1');
});