Use webmock instead of test double for testing

A few bugs crept into the `support` app as a result of using a dummy implementation of the Zendesk client (which didn't mimic the behaviour of the real thing closely enough). Since the [Zendesk API](http://developer.zendesk.com/documentation/rest_api/tickets.html) is pretty predictable and follows REST conventions fairly closely, this PR introduces mocking at the HTTP level (using the `webmock` gem). As a result, the `DummyClient` class can be really slimmed down to the bare minimum (4c0e3be). Some test helpers are introduced (mimicking the `gds-api-adapters` gem).

This PR also better encapsulates the `GDSZendesk::Users` class better by exposing it through the `GDSZendesk::Client` class (see dd09405).