Make Public TwilioRestClient Methods Virtual

When writing an app that has TwilioRestClient as a dependency, it can be difficult to unit test because TwilioRestClient does not implement an interface, and the vast majority of its methods are not virtual.

This pull request adds the virtual modifier to most public methods of TwilioRestClient (with the exception of the WinRT project, which I could not load) to allow subclassing and mocking of TwilioRestClient.

I also cleaned up a wayward .DS_Store file left over from someone's Mac.