add names for five numerics; fix handling of 002 and 003

* add names for five numerics (names taken from [RFC 2812 § 5.1](http://tools.ietf.org/html/rfc2812#section-5.1) and [IRC/2 Numeric List](https://www.alien.net.au/irc/irc2numerics.html))
* fix handling of 002 and 003 -- their cases previously fell through to 004's case, now they're properly ignored along with other ignored numerics