Fixes undefined usernames in Slack's IRC gateway

Slack users can have dots in their usernames and raw messages look something
like this:

    :dave.test!dave.test@irc.tinyspeck.com PRIVMSG #hubottest :testing

This patch adds the dot to the message prefix matching to properly parse out the
username.  Undefined usernames can cause all sorts of problems with Hubot
recognizing commands.