Tropescraper

Fixes issues related to documentation and package generation:
- Installed library through pip was not working due the lack of sub-modules explicitly listed in setup.py.
- New long description in the package copied from the README (AKA, visible in https://pypi.org/project/tropescraper/)
- Improved README.

**Release notes**:

This version is cleaner, automatically tested, more reliable and more maintainable.

Includes:
- redesign of classes according to Clean Architecture.
- refactor of the code according to Clean Code.
- better test coverage of the three adaptors:
- file-based cache.
- web page parser.
- web page retriever.

Updated in PyPI: https://pypi.org/project/tropescraper/