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
Unless this function is likely to be needed outside I suggest just a function using `isinstance`.  I think that that's easier for most developers to grok and is IMO, a bit simpler.  Just my opinion though, feel free to keep as is if you disagree.