Support flow-add and flow-remove events

Part and remove channel on flow-remove event
Add and join channel on flow-add event
Minor tuning
Fix handling of add-people action

We can't use the list of nicks that comes with the event as it might
contain nick conflicts. Instead figure out which users were added from
the channel data after conflicts have been resolved.
Properly resolve nick conflicts when adding new flow
Merge pull request #39 from flowdock/flow-add-remove

Join new flows, part from blocked ones