Add a ChangeExecEventListener to UpdateVisitor and RollbackVisitor allowing for finer grained monitoring of change executions.  This could be useful for progress monitors, enhanced logging/debugging, etc.
changeExecEventListener: add willRun method to listener api
changeExecListener: decided I liked fireWillRun and fireRan better for method names
Merge pull request #178 from skelter/changeExecEventListener

Change exec event listener