#90, #103: Pytest introduction, coverage report and badge generations

• Introduce pytest{,-cov,-sugar} development dependencies
• Change .gitlab-ci.yml and Makefile test rule
  • Install dev dependencies into the tests jobs to install pytest
  • The tests jobs take a bit longer
• Generate coverage report web pages and badge with GitLab feature
  • The badge can be found here for this branch.
• Change tests jobs names using a colon
• helpers tests were not recognized without the test_ prefixes and __init__.py
• Update and refactor Tests section with pytest usage
• Fix api.ws2p.test_ws2p.test_document_message() test

Close #90 (closed), #103 (closed).