- updated readme with section on testing and setting up for E2E
- automated create/update of forms and submissions for E2E tests
- copied over md_table functionality from pyxform to allow specifying
  test forms as markdown instead of accumulating XLSX files
- added openpyxl as a dev dependency to support conversion of md_table
  XLSForm data to XLSX format
- updated test_client.py / TestUsage.test_direct to use the new
  `$select` parameter, also showing how to select sub-properties
- fixed percent-encoding in `session.py` to match Central behaviour,
  see new docstring comments for details, update tests accordingly.