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.
Are these built up just once, or would it make sense that we expand this class so we can pass the 4 ser types around and then just reify into the map at a final toMap step?