Python3 support and more, with backward-incompatible changes

• bump version
• improve (a bit) the documentation
• makes pylint happier
• test: do not run as root
• Python3 + 100% code-coverage (fix #6 (closed), #7 (closed))
• retab...
• docs: link to code coverage
• docs: use version from module
• update version, incompatible changes ahead
• docs: change theme