Include license file in the generated wheel package

The wheel package format supports including the license file. This is
done using the [metadata] section in the setup.cfg file. For additional
information on this feature, see:

https://wheel.readthedocs.io/en/stable/index.html#including-the-license-in-the-generated-wheel-file

The wheel now complies with the license:

> The above copyright notice and this permission notice shall be
> included in all copies or substantial portions of the Software.

doc: improve section about usage of "noqa"

Make pytest-runner an optional dependency

Rename [wheel] section to [bdist_wheel] as the former is legacy

See:

https://bitbucket.org/pypa/wheel/src/54ddbcc9cec25e1f4d111a142b8bfaa163130a61/wheel/bdist_wheel.py?fileviewer=file-view-default#bdist_wheel.py-119:125

http://pythonwheels.com/

Minor README tweaks and typo fixes

This adds some missing words, fixes some typographical errors, and adds
some missing links to describe concepts mentioned in the README.

Make signatures of default method match

PathGraphingAstVisitor inherits from ASTVisitor, but they have different signatures.

Test against python3.6

Have mccabe consider all statements as statements

This allows tools like Flake8 to report a column number (that is not 0)
for users who care about the column number (in the case of the bug
report a tool that integrates with a text editor).

Related-to #43

Fix ValueError: read of closed file

By default, McCabe's max_complexity value should be the command-line
default.

Handle files with BOMs

This emulates how Flake8 does it for us in a simplified way.

Closes gh-39

Add support for PEP 0492 keywords