expenditure from commitment. This requires a set of bank holidays which is retrieved using
retrieve_bank_holidays().

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.

This PR implements a fuller calculation of on-costs. It is *highly*
recommended that you read the accompanying documentation for this commit
as it is quite in-depth.

Closes #7.