add version requirements to Event classes

This first submission is a Request For Comments, I still need to write tests before I ask to add this branch.

While reviewing SignalEvent, I realized it would not work for the tor version I am running.

ProtocolError: SETEVENTS received unexpected response
Unrecognized event "SIGNAL"

I thought the error message could be more meaningful (e.g.

ProtocolError: SETEVENTS received unexpected response
SIGNAL event requires Tor version 0.2.3.1-alpha or later

So, this patch series adds a version check to Controller.add_event_listener() and provides versions to Event classes (with a default version in Event itself).

Is this something we should add to Stem? Is this the format to use? I'd appreciate any feedback.

Pull the exp-event-versions-v1 branch from git://gitorious.org/stem-robinson/stem-robinson.git or view commit logs @ https://gitorious.org/stem-robinson/stem-robinson/commits/exp-event-versions-v1

Trac:
Username: robinson

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