Clean up the streamer.onmessage handler

- Remove multiple checks to ensure that payload is an array, the
  backend only sends arrays[1][2].
- Convert python style method docstrings to coffeescript comments.
- Use interpolation for the user string.

[1]: https://github.com/hypothesis/h/blob/master/h/streamer.py#L562
[2]: https://github.com/hypothesis/h/blob/master/h/streamer.py#L488