Allow for tracing of execution - using NodeEvaluationEvent

[bhouston]

I've written a means of listening to the execution flow via GraphEvaluator.onNodeEvaluation. It fires off NodeEvaluationEvents which you can then use to highlight nodes in the graph.

I think that maybe this should be paired with a "step" advance function rather than execute to end or something like this? Like a step-debugger?