irc.py: allow dots in nicknames
irc.py: allow dots in nicknames

psyced uses that:

    >>> irc.Prefix.parse('xmpp:grawity@example.com!grawity@example.com')
    <IRC.Prefix: 'xmpp:grawity@example.com' ! 'grawity' @ 'example.com'>

grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks,grawity/hacks