Updated to use v3.0 of the BrowserStack API.

- Updated the objects and functions to match the new JSON structures
getting returned by the API.
- Added method to get API status.
- Added unit test for API status call.
- Changed web requests to use WebClient instead.
- Return the time used (nice for unit testing) when terminating a
worker.
- Updated JSON.NET version and applied it as a Nuget package.
- Added package restoring.