extracting Zendesk client from the 'support' app
adding a dummy Zendesk client
adding a development mode

Adding a stub implementation for testing.
Additionally, since Zendesk doesn't provide a staging environment, it's
important that we don't raise real tickets during exploratory testing.
integration Users class from the support app
logging user creation failure in dummy client
Fixing typo in README
Improved development mode description in README
README improvements
dummy client: more ways to simulate failure

Adding ability for failure to be triggered by a specific comment
in the dummy client. Useful for exploratory testing.
Improving the README
Merge pull request #4 from alphagov/extracting_from_support_app

Extracting common code from the 'support' app