Sub-Network support

I am a big fan of the NoFlo project and would love to see it become an even bigger success.

I was excited to see the existing Python run-time prototype and have attempted to do what I can to keep it moving forward. 

To that end, I offer this commit in an effort to support the following features:
* run graph process nodes in parallel
* allow sub-networks (or composite-components)

I welcome feedback, questions and concerns.