Add support for code quality metrics.

Currently just pep8 and pylint, though it can be extended to other
checkers.
Fix failing tests due to refactoring.
Fix test
Remove unused method.
Add tests for arg parsing
Beginnings of integration testing
Add violations test file
test
Violations files
Add expected pep8 violations output and fix up mocking of Popen in tests.
Add violations tests; update test requirements
Fix Pylint checker regex and tests.

By default Pylint doesn't show error codes, just types (C, E, W,
etc.). Codes are turned on in the pylintrc for edx-platform, though,
so we should be able to handle them if they're present.
Test console output.
Update violations testing