Python-graphs

1. Adds a module frame to catch raises in top-level code.
2. Also marks except expressions and finally blocks as branch points.

The branch kinds are: branches, except_branches, and reraise_branches.
* branches are your usual branch decisions: ifs, fors, and whiles.
* except_branches are at "except E:" statements, with True indicating the exception matches and False indicating it does not
* reraise_branches are at the end of "finally:" blocks, with True indicating the path taken after finally if an error has been raised previously, and False indicating the path taken if there's nothing to reraise at the end of the finally.

Includes \_\_init\_\_.py files to allow use of python_graphs library from Python 2.

Initial public release of the python_graphs library.

Core features:
- control flow graph generation
- data flow analyses
- program graph construction
- cyclomatic complexity calculation
- a solid test suite for all the above
- visualizations using graphviz for each of the graph representations