Implement pusher_pong.
Revert "Implement pusher_pong."

This reverts commit b947efa1ed473ca1e67d22f79d5e459e88e24f4b.
Just response with 'ping' message with 'pong'.
Merge pull request #124 from Juanmcuello/pusher_pong

Implement pusher_pong.