improve graphviz output. Create thrift command for getting graphviz output.

Summary:
You can now use the python client to get the graphviz representation of the
graph.

In addition to that, the graphviz output now contains the rules as well.

Test Plan: Run falcon.py -v in order to try and get the graphviz output.

Reviewers: NicolasDP

Reviewed By: NicolasDP

Differential Revision: http://91.121.199.76/D35