tornado's ioloop intergration

Sample usage:

from pyxmpp2.mainloop.tornado import TornadoMainLoop
Client(jid, [self, version_provider], settings, main_loop=TornadoMainLoop())
add docstrings and fix some warnings by pylint
fix a bug caused by previous cleanup
Merge pull request #76 from lilydjwg/tornado_mainloop

tornado's ioloop intergration