In #118, there was a change made to rename all commonmark imports from `commonmark` to `CommonMark` due to a breaking change to the package name in that dependency in version 0.8.1. However, the dependency on `commonmark` wasn't updated, which means that people pulling in commonmark >= 0.7.3 and < 0.8.1 are experiencing issues while using recommonmark. This updates the versioned dependency accordingly to resolve this issue.
Add a description, URL and license to `setup.py` so [the recommonmark package page on PyPI](https://pypi.org/project/recommonmark/) will include that information.
This was already dealing with a rewrite due to the spec change on CommonMark, so
I took the opportunity to modernize the code and clean things up as well.
* Updates to support CommonMark==0.7.3
* Reworks pattern for handling CommonMark nodes, mimic docutils visit/depart
pattern
* Drops SectionHandler class, as it was only used in one place. Merge into the
parser class, where it is used
* Drops requirements file for setup.py config, as this is a package requirements
is superfluous
* Adds a number of more detailed test cases
* Adds test runner for 3.5, 3.6
* Updates strictness on prospector, cleans up linting errors and docstring
problems
