Rdiffb

* Drop python 2.7, build against 3.6, 3.7, 3.8

* Drop python 3.3 from tests: 2.7 and 3.4+ only

* Add `compare_graphs` method
* Improve test coverage

* Significant refactoring
* Rename command line tool from `rdiffb.py` to `rdfdiffb.py`
* Ditch python 2.6 tests (because of `rst-lint` issue), focus on 2.7 and 3.3+
* Uploaded to pypi as <https://pypi.python.org/pypi/rdiffb>

* First working version...