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
@Astro03  I don't plan on implementing this right now. I put this in as a marker to imply that the api would allow us to pass config ids in this method should we plan to implement it.

I can remove the TODO if you want?