ENH: release gil for indexing operations

closes gh-4310
ENH: release gil for put and putmask
DOC: specify that 2d array must be non-empty in np.poly error message.

Fixes #2092. Changes message passed to ValueError.
Merge pull request #4067 from hmeine/master

explain character codes of dtype.kind
Merge pull request #4498 from meltingwax/meltingwax/2092

DOC: specify that 2d array must be non-empty in np.poly error message.
Merge pull request #4497 from juliantaylor/index-gil-release

ENH: release gil for indexing operations