Port to Python 3

Enter the future! Use Python 3!

It seems that all dependencies except for Twisted are fully ported to Python 3.  The core of Twisted is ported (as of version 14), but ``twisted.words`` is not.  This leaves us with a few options:

* Stay on Python 2 - boring!
* Port ``twisted.words.protocols.irc`` to Python 3 - this is a pretty big job, and people have already tried and failed (http://twistedmatrix.com/trac/ticket/6320).
* Port csbot onto a different framework.

The latter seems the most viable.  With Python 3.4 and "project Tulip" making it into the standard library as the ``asyncio`` module, it's the logical target.  Either we should write a new client library on top of ``asyncio`` or base csbot on an existing one, e.g. https://github.com/gawel/irc3.
Either that, or removed entirely.  Nothing currently uses this information, it would only be useful if in a future authentication-style plugin we wanted to track the modes that users have in the channels.  It depends on the IRCClient parsing and storing server configuration information that is sent around the same time as the welcome, which should probably be done for other reasons too (maximum line length, etc.).