Makes the lib line up more with the irc rfc in message vs args handling
Replaces the Event.Message field with a function

Note that this purposefully breaks backwords compatibility. Because of
how Arguments are now handled, this will force users using this library
to update anything using this.

This is not bad, as it seems, because otherwise (just updating the
Event.Arguments to include the Message) the change could cause silent,
strange breakages.