WIP: Chunk CSVs using pandas.read_csv

closes #587

- [ ] performance diffs
- [ ] newline robustness (e.g., appending to the end of a file that has an existing newline)
- [x] revert dshape inference changes and don't let pandas turn `''` into `nan`
i know you keep telling me that ... trying to use `toolz` and `operator` module things a bit less.