Removed unimplemented client Handler type/method.

Removed beginnings of Handler type, which I assume to be the
beginning of an event driven system for handling incoming messages, so
that you can actually use the irc.Client interface with NewClient (before the type client didn't satisfy the Client interface so it was useless).

I'm not sure a library like this needs events. As it stands go-irc is a simple client that doesn't impose a style on users; handlers and callbacks would force you to be event driven.