Add coverage package

It is being used by run_tests
Integrate with coveralls
Merge pull request #7 from alphagov/add-coverage

Add coverage package
Add status badge for coveralls
Merge pull request #8 from alphagov/add-coveralls-badge

Add status badge for coveralls
Improve docs for running in dev with signonotron2
Add basic user session
Add list of data sets to index page

- This uses an editable pip target for performanceplatform-client, this
  needs to be changed when the AdminAPI is released.
Set the testing flag in tests

This propagates errors, makes debugging tests easier.
Use version 0.0.6 of performanceplatform-client