added integration for Configurations

Added `GetConfigurationGroups` method to client (http://docs.gurock.com/testrail-api2/reference-configs)

Added ability to pass config_ids when creating a test plan (http://docs.gurock.com/testrail-api2/reference-plans#add_plan_entry)

`Plan` object now contains list of `Run`s which are populated on calling get_plan