add GitLab CI configuration
Add our standard GitLab CI configuration and tweak the project to be CI friendly:
- A simple Dockerfile is added which builds an image which can run tox.
- The tox.ini is updated to save JUnit reports where our CI expects to find them and to add coverage XML reports.
- Remove the Circle CI badge from the README. We have GitLab CI badges set up for the entire UIS group in GitLab so we don't need any additional configuration.