RFC/DOC: Standardize the column index type
In [ARROW-1976](https://issues.apache.org/jira/browse/ARROW-1976) a user hit
an issue with serialization of unicode column index values using Python 2.

This PR proproses to make all column names into whatever the type of
`six.text_type` is, therefore handling Unicode in as uniform a way as possible
given the terrible state of Unicode in Python 2.

cc @wesm @TomAugspurger @jorisvandenbossche

I considered doing this by adding another field to indicate the individual type of the name, but that adds quite a bit of technical debt and backwards incompatibility. Since `json.loads`/`json.dumps` can read unicode strings across Pythons, this seemed like a backwards compatible way to support unicode column names across files written in different versions of Python (and pyarrow).