Define "unit-testability" rules for clients and data objects

[pakrym]

Our client types should support being decorated/mocked by customers to be able to unit tests their code without running against a live service or having to wrap our type to make them unit-testable.

Couple things that come to mind:

0. There should be a way for customer to avoid all network comunication
1. Client methods should be virtual or available on an interface so they can be overriden
2. Data objects should allow setting all properties (via protected setters or a different set of constructors) to allow simulating server responses

Another solution to this is to tell customers to replace the transport and simulate the services on HTTP level but this might be very complicated because it involves capturing real responses and dealing with protocol details.

Related issues:

Azure/azure-sdk#18

Azure/azure-storage-net#465

#3968
#6

https://github.com/Azure/azure-storage-net/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+mock

cc @maririos @KrzysztofCwalina @AlexGhiondea