pip installation / setup.py

@tomoleary, it'd be nice to be able to pip install `hippyflow` instead of having to create an environment variable to find the library. 
1. How much can we copy from `hippylib`'s [`setup.py`](https://github.com/hippylib/hippylib/blob/master/setup.py) and [`INSTALL.md`](https://github.com/hippylib/hippylib/blob/master/INSTALL.md)? 
2. What other dependencies & optionals would I need to add?