fixes the issue where users are not tracked correctly on some IRCd's

The current code assumes a NICK line looks like this:

    :old_nick NICK :new_nick

[RFC 1459](http://tools.ietf.org/html/rfc1459#section-4.1.2) says that a NICK line should look like this:

    :old_nick NICK new_nick

To deal with some IRCd's using the first (e.g. Snoonet, AFnet) and other IRCd's using the RFC 1459 method, I have added a check to see if new_nick begins with a ":", and remove it if so. Otherwise, the nick is left untouched.

This allows for tracking of user aliases on nick change on any IRCd.