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.
@ianoc no, I didn't try with 2.10.

It was wrapping it like:
```scala
this + x
+ y
```
rather than
```scala
this + y
  + x
```
Maybe neither would work, though.