Adding PING/PONG

This PR adds PING and PONG. Tested with autobahn testsuite. Passes same tests as previous commit plus 2.* and a few others.

I added emit('pong', $frame) when a PONG message is received. Not sure if this is the best solution.

Also changed WebSocket::send to be able to take a Frame as the argument (so a PING can be sent).