Fix voice call flow deserialization and other unrelated clean-up

[codefrenzy]

This pull request includes a few changes:

• Changed the target framework for the MessageBird project back down to .net framework 4.0 and netstandard2.0. This allows the solution to build on Mac.
• Changed the return type of create, update and view methods for voice call flows to return the same as the Java SDK does (a data array with a links object). Added more assertions to unit tests to verify properties are deserialized properly. Changed the namespaces to the default one for all the unit tests.
• Renaming existing namespace MessageBird.Objects.VoiceCalls to MessageBird.Objects.Voice and MessageBird.Resources.VoiceCalls to MessageBird.Resources.Voice so that we can put all other Voice API related objects and resources into these namespaces together.
• Also renaming existing VoiceCallFlow classes to CallFlow, to mimic the PHP SDK and eventually allow us to split the MessageBird client into separate clients per API.