Added Attendee model and associated unit test class.

Attendee model focuses on the personal information contained in the API response. Meeting information could (should?) be extracted separately.
Implemented AuthService unit test. Only testing success behavior.

Starting to refactor Client-Service relationship to be more test-friendly. The Client now handles sending the requests; Services only configure each request.
Refactor services to use `Client::sendRequest()` instead of `AbstractService`.
Initial `GroupService` test class.
Added `GET` methods for the organizer service and corresponding positive unit tests.

Get organizers by group API call added to `GroupService` because information maintained at group endpoint. May be a candidate to move later.