Validate nick according to the IRC RFC 2812 spec.
Reference: http://tools.ietf.org/rfcmarkup?doc=2812#section-2.3.1
Some irc clients' tab-completion feature appends 'address' characters
to nick, such as ':' or ','. We try correcting for that by trimming
a char off the end.
If nick incorrigibly invalid, return False, otherwise,
return (possibly trimmed) nick.