Kill module

Not sure it's the expected behaviours as it's differs from the RFC but it's compliant with flabby's ticket.

Restriction:
- Op only.

Usage:
!kill nick

Action:
First call to "kill" will put the user in a list and kick him from current channel.
Every time the user will try to come back, he'll be kicked in chain.
Second call to "kill" will remove that user from the list