Add a Config class to make configuration understandable

This is an attempt to unify the jank in summingbird-scalding, internal to Twitter, and in the Job code.

Next up, a clean way to start and run jobs without a Job class.
1. Do we not want to return a Config?
2. Do we want a method configToHadoop that then will set everything in the map to Hadoop? Though maybe that isn't necessary since internally we just require Maps