Adding convenience methods to Sentry to hide Group/User/Throttle Providers

Rather than calling ```Sentry::getUserProvider->findByID(1)```, I'd like to call ```Sentry::findUserByID(1)```.  I don't need to know that Sentry uses Group/User/Throttle Providers -- this change hides that implementation knowledge from programmers.


This commit updates the docs and tests as well.