Remove trackTimings from WebSocket, fix it for WAMP.

This pull request isn't ready to be merged; I'm opening it up to gather some feedback and ask a few questions.

I was attempting to add logging to my WAMP1 server and found that trackTimings was completely non-functional.  So I dove into the code and fixed it.  I've done/will do the following:

* Got trackTimings to work for WAMP1, which includes the events `onBeforeCall`, `onAfterCallSuccess`, `onAfterCallError`, `onAfterSendCallSuccess`, and `onAfterSendCallError`.
* Remove trackTimings for raw WebSocket events.  I did this because timings are relative, and since messages can be sent and received at any time, what's the timing relative to?
* `autobahn.websocket.protocol.Timings` and `autobahn.utils.Tracker` are duplicates.  Can I get rid of  `autobahn.websocket.protocol.Timings` in favor of `autobahn.utils.Tracker`?
* Make `Tracker.__getitem__` return a unix timestamp, so I can get an absolute time of when an event happened.

Are all these changes okay?