Add replacing of buddy names with @nick on send

This allows you to use pidgin's normal autocomplete to write out the names of people in a hipchat chatroom, but when you send the message have it sent using the appropriate @nick-style nicknames so that those users will be notified in the standard hipchat way.

This is a rather substantial change, so if you have any objections to the approach or anything other thoughts or comments, well -- that would all be completely understandable. :-)

--Chouser