Streamer.onmessage now receives parsed JSON
Clean up the streamer.onmessage handler

- Fix "single player mode" filtering of highlights. It was assuming
  that $scope.persona was an object rather than a string.
- 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.

[1]: https://github.com/hypothesis/h/blob/master/h/streamer.py#L562
[2]: https://github.com/hypothesis/h/blob/master/h/streamer.py#L488
Merge pull request #1804 from hypothesis/parse-streamer-json

Clean up Streamer.onmessage