emit a leavemessage on kick event

a 'part' or 'quit' event is not emitted when a user is kicked, so i just replicated the logic from similar listeners.

without this, my irc bot isn't able to purge users that are no longer present from its memory. when i say 'memory', i don't mean robot.brain but rather the data-structure my bot creates, itself.

i'm not sure if i've gone about this the right way. also, i'm not even sure if this is entirely necessary. perhaps it's _already_ possible to catch a 'kick' event. if it is, please accept my apologies. :P