Motd refactoring

* MOTD contents are now read from a file that can be easily updated
* Moved the sending of MOTD and other registration messages from PONG to the authentication helper
* Delayed the sending of registration messages when PASS authenticating
* Added support for MOTD command

Additional changes/comments:
* Send the common "We are processing your connection" message once connected, so it doesn't feel as it got somehow stuck if the PASS authentication takes a little while.
* Understand commands sent in lowercase
* PONG now just stores the current timestamp that can be used when we implement a periodic timer to do pinging. It could just be removed for now, though.