Refactored Subscription related cmdlts and added tests

* Refactored Account (2100040), Subscription (2100044), and Environment (2100050) cmdlts
* Added tests for all public methods on ProfileClient
* Added Add-AzureSubcription cmdlt
I would keep them separately to make them feel more c#ish. Also AzureSubscription return object is not applicable for `void AddAzureSubscriptions`