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.

Finally, I found a defect with the RESTSharp client timeout - it is currently set to 3.05 seconds, and I think the intent of the change was to set it to 30.5 seconds (slightly larger than the Twilio timeout of 30 seconds).