WIP: Implement ColumnWise for MongoDB

tests:

- [x] for more than just a single `+` op
- [x] possibly implement the [available gamut of mongo arith ops](http://docs.mongodb.org/manual/reference/operator/aggregation-arithmetic/)
- [x] fix `Summary` to work with arith ops
Fantastic doctest!  This actually fixed my previous flawed understanding of what this function did.