WebSocket Factory (backed by Web Workers)

This PR aims to add a useful Angular **WebSocket Factory**:

```JavaScript
angular.module('myApp', [])
.controller('WebSocketCtrl', function($scope, WebSocket) {
  var ws = new WebSocket('ws://localhost:3000');

  ws.on('new_message', function(data) {
    // Do something with data...
  });
});
```

*Server-Side*, the WebSocket Library should send a message to the client like this:

```JavaScript
...
ws.send(JSON.stringify({ type: 'new_message', ...}));
...
```

Each new WS instance runs into a separate **Web Worker**.

I still need to fix some compatibility issues (works only on Chrome for now), add some tests and documentation.