ENH: check for __array__ instead of ndarray subclasses when creating an Index

This allows custom ndarray-like objects which aren't actual ndarrays
to be smoothly cast to a pandas.Index:
https://github.com/pydata/pandas/issues/5460#issuecomment-44474502
DOC/RLS: remove duplicate content in release.rst and move remaining to whatsnew
Merge pull request #7270 from shoyer/use-__array__

ENH: check for __array__ instead of ndarray subclasses when creating an Index
Merge pull request #7275 from jorisvandenbossche/release-notes

DOC/RLS: remove duplicate content in release.rst and move remaining to whatsnew
DOC: add thanks list to release.rst
RLS: 0.14.0 final
DOC: add whatsnew / release for 0.14.1
BUG: grouped hist raises error with single group