Fix CONN_BW event listening

The ConnectionBandwidthEvent overwrites the type field of the Event class. Therefore the method _handle_event in control.py erroneously checks if the connection type (if it is a ConnectionBandwidthEvent) is contained in the event listeners list. All in all you can't receive CONN_BW events in stem. I fixed it by renaming the type of the ConnectionBandwidthEvent to conn_type.

My changes can be found here: https://github.com/kithack/stem/commit/23fe7b6cbd69c6209657c9531a21502a67ecf5e5

Trac:
Username: timonh