Implement the `csv -H` option properly

This is basically part 1 of @davidmiller’s [weird html output fix](https://github.com/okfn/datapipes/issues/35#issuecomment-24048988).

The only bit I have changed is that non-header rows are always zero-indexed. I just did that because it was easier (required fewer changes to existing operators).

Fixes #35.

EDIT:
“weird html output fix” is unintentionally ambiguous.