Add send_channel_event method on socket for client channel events

This gem appeared to have no way to trigger client events, so I added a method on the socket class called send_channel_event(channel, event, data) to allow client- and presence- events on private and presence channels.

send_event could not be used because the channel specification needed to be at the top level of the json message as per the pusher protocol at http://pusher.com/docs/pusher_protocol#channel-client-events.