Default endpoints

@jkogara - thanks for all your work on this, looks great!
I was thinking it would be great if we could bring the URL handling in this project closer in line with the standard Swrve SDKs which give a custom endpoint per game_id, which allows the Swrve ops team extra control for routing traffic.

This pull request currently fails your rspec tests because it raises an exception for an underspecified config but it along the lines of what I was thinking. What do you think?