- add instrumentation events

- add instrumentation events
- simplify listen_for_events example with new system
- add "propagate", "retval", "raw" flags to attribute events.  this solves the "return value"
issue as well as the "subclass" issue.
- begin thinking about event removal.   Each listen() method will have a corresponding remove().
Custom listen() methods will have to package all the info onto the event function that is needed
to remove its state.