Add Spec for Server has_monitored_service? Method

Add tests for the Server has_monitored_service? method.
Add Server has_monitored_service? Method

Add the has_monitored_service? method to the Server class to return
whether a specified host has the passed service being currently
monitored.
Update README for has_monitored_service? Method

Update the README to include the Server has_monitored_service? method
information.