basic capability negotiation

- multi-prefix is supported as an example
send proper replies for cap protocol
Merge remote-tracking branch 'origin/master' into cap-protocol

Conflicts:
	irc/server.go
Merge pull request #8 from jlatt/cap-protocol

basic capability negotiation
Merge remote-tracking branch 'origin/master' into user-mask

Conflicts:
	irc/reply.go
	irc/server.go
	irc/types.go