Fix block on PING request

This patch fixes the indefinite hold of the lock following a PING message

The _raw_recv and _mcon now respond to a PING message like all the other messages and will not block
This blocking would prevent the bot from performing "send" (and other) requests until activity unblocked the bot

This fix moves the PONG answer to the recv function where all the other messages processing takes place and returns the None event (no further processing required)

Fixes #33