- Added remaining tests for new server calls.
- Added explicit Accept header of 'application/xml' for the API client - using the default wildcard Accept
  generated by requests breaks the delete_server api endpoint (have raised a support ticket with newrelic as think this is a bug)
- Added fixture data for reponses.
- Updated excepions so we don't unecessarily print output to the screen + pass exception messages through
  in baseclass