Add :ping and :pong to Phoenix.Socket.Handler.websocket_handle/3

The `cowboy_websocket` module takes care of sending the reply to ping
messages, but then immediately crashes the process by invoking
`websocket_handle`. This change just adds no-op clauses
appropriately.

For future reference, Cowboy also provides a `:binary` tag, which is
currently unhandled.
Merge pull request #449 from nickmeharry/websocket-ping-pong-frames

Add :ping and :pong to Phoenix.Socket.Handler.websocket_handle/3