Support for multiple subscriptions on the same topic

[balek]

Each subscribe call rewrites handler function for topic now. There is no way to call multiple handlers without some middle layer.

session.subscribe(handler1, 'test_topic')
session.subscribe(handler2, 'test_topic')

[oberstet]

Correct. I once contemplated about doing that in the Subscription object returned by subscribe.

[tardyp]

I have a fix for this:
tardyp@e58a633

But this is based on the pep8 branch. Please merge it so that I can contribute it! :)