Adding Stomp based connector and async workflow runner.

This adds a gateway to connect and publish plans and steps across
the Stomp protocol. Also adds an asynchronous service for running
a worker process that can consume and publish messages for executing
an execution plan.

The workflow example in examples/ can now be run in serial or stomp
mode. To run in serial mode just execute 'ruby ./bin/scenario_runner.rb'.
To run asynchronously using Stomp, ensure you have a Stomp broker running,
and Katello checked out at the same level as your dynflow library. Start up
the process manager with ./bin/dynflow.rb and then execute the scenario
with ./bin/scenario_runner.rb stomp.