Before: 1_000_000_000 = "miliard milionów tysięcy" (with words for zero millions and thousands)
After: "miliard" (no words if value for given power of 1000 is 0)
* Activate flake8, isort and coveralls in the configuration of tox
* Update code to respect PEP8
* Fix bug in the lang_IT for python 3
* Update the README to include the new converters.