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
What about combining both in this?
```
public AzureSubscription AddSubscription(params AzureSubscriptions[] subscriptions)
```