Added mocking with minimock to tests
Added test dumps

- dumps of rpm and dpkg output
- dumps of swid and software-id output
Added ElementTree as fallback import
Added exit codes

Codes are documented in README.rst

fixes https://github.com/tnc-ba/strongTNC/issues/104
Reorganized imports

- Added future imports
- Correct grouping
Moved common code from environment subclasses to CommonEnvironment

- `get_os_string` is already in CommonEnvironment and does not need to
  be redefined.
- Moved `is_file` method to CommonEnvironment.
- Converted methods of environment classes from `staticmethod` to
  `classmethod`.
- Added or corrected many docstrings.
Fixed dangerous kwarg default

Never use mutable objects as kwarg default.