Metrics integration

Add optional nflow-metrics project that provides integration to http://metrics.codahale.com/. MetricsWorkflowExecutorListener plugs into WorkflowExecutor and records statistics about various performance aspects. Metrics data are exposed in JMX if Spring profile  "jmx" is enabled.

Adds mechanism to add additional Spring beans to nflow application:

        new StartNflow().registerSpringContext(NflowMetricsContext.class).startJetty(7500, "local", "jmx");