Add Durable Entities support

[andystaples]

Add support for durable entities to this SDK

See the included documentation/samples/tests for usage guidelines.

Reviewers: Request special attention to file structures, naming, docs and tests

Discussion points:

• Optional parameters to some of the entity requests, like start time and cancellation tokens - should these be implemented now or later?
• Typing for entity calls - look at TInput/TOutput for activities, compare with entities (abstraction because of EntityInstanceId)
• Class-based entities - should we have the object state represent the entity state, or preserve the get_state & from_state behavior here (research into the dotnet SDK required)