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
i don't think the refactoring will be difficult, less sure about the simplicity but i suspect that it will make things simpler to not have to remember when to use `scalar_symbol` vs `expr`.