Adds failing test for nominal arff reading

The arff io reader fails to load arff files containing
a single nominal value of only 1 character.  Adds a
test demonstrating the failure in this case.
Fixes failing test of arff reader

Alteres the regex for parsing nominal values in
arff files so that single character nominal
values can now be successfully parsed.
Fixes arff io test name
DOC: add 0.13.1 release notes.

Conflicts:
	doc/source/release.rst
Merge pull request #3073 from ghl3/arff_nominal_fix

Arff reader with nominal value of 1 character
BUG: fix range keyword for stats.binned_statistic.  Closes gh-3063.
TST: clean up stats.binned_statistic test for range kw a bit.
Merge branch 'pr/3064' into master.

See https://github.com/scipy/scipy/pull/3064
TST: clean up some tests in the io module.
Merge pull request #3072 from rgommers/fix-coveralls

MAINT: fix coveralls.io reporting.
DOC: complete stats.boxcox docstring.  Add add boxcox functions to refguide.

Also do a PEP8 cleanup of `boxcox`.
TST: add tests fox stats.boxcox.
DOC: complete stats.boxcox_llf docstring.  Also PEP8ify that function.
TST: add tests for stats.boxcox_llf.  Also add some input parameter handling.

Add also a reference to the original paper of Box and Cox.