This updates Flake8Tool to subclass the newer BaseTool, and to

use the newer features it offers (dependencies, improved centralized

base command building, and standardized error code/column output).
The reporting actually parses the CodeClimate format. Flake8 supports

plugins for reporters, and one of them, flake8-json, offers

CodeClimate output. This is a format that many other tools support, so

it makes sense to use it wherever possible. A new utility function has

been added to convert these payloads into comments.
We unconditionally depend on flake8-json for the CodeClimate support,

even if flake8 itself isn't installed. This is a small dependency, and

ensures we don't need to worry about having users install yet one more

thing.

Unit tests pass on Python 2.7 and 3.x.