Add AbstractModel class with response helpers
Return CaseModel from UpdateCase command

Desk returns the newly updated case with the result, so we should offer
this as a return value rather than making the user issue a GetCase
request if they want the newly updated case.
Add createMockCommand() method to base test class

Creates a mock OperationCommand with a mock Response set, containing
particular data.
Add empty ContentClient
Add TopicModel
Add GetTopic command
Enforce "count" and "page" param limits
Add TopicArray model
Add GetTopics command
Add iterator for GetTopics command
Add CreateTopic to service description