* Construct CartAPI URL from OS environment variables

* Fix environment variables prefix for CartAPI class

* Add logger debug statements to CartAPI instance methods

* Remove unused import

* Revert changes to Downloader class

Commit 3449fb7a3b2d821fcfc40b89d2513ca6b6f85ff3 did not resolve merge conflicts.

* Resolve pylint "protected-access" warning

* Resolve pylint D200 and D401 warnings

* Resolve pylint pep257 warning

* Resolve flake8 F401 warning

* resolve autopep8 warnings

* CartAPI response is not JSON data

* Add coverage test for CartAPI URL

* Resolve pylint "protected-access" warning