Add authentication of subscription to a channel in the Pusher Controller.
Add authentication of subscription to a channel in the Pusher
Controller.

This implements both presence channel authorization (used for the river)
as well as private channel authorization (used for private messages).

kimchanyoung/nghborly,kimchanyoung/nghborly,nyc-mud-turtles-2015/nghborly,nyc-mud-turtles-2015/nghborly,kimchanyoung/nghborly,nyc-mud-turtles-2015/nghborly