Add integration to metrics library http://metrics.codahale.com/

WorkflowExecutorListener mechanism is used to gather statistics
about time it takes to process states, number of retries, and
delays in processings. Adds project nflow-metrics that is. Usage
of nflow-metrics is optional, it is not automatically included as
a dependency.
Merge pull request #10 from NitorCreations/nflow-metrics-integration

Add integration to metrics library http://metrics.codahale.com/