Add doctest to tox.ini

[tkem]

Need to refactor code samples wrt dict ordering first.

[tkem]

See also http://tox.readthedocs.io/en/latest/example/unittest.html

[tkem]

Using Sphinx doctest could circumvent some issues with Python 2.7, since all the doctests should now target Python 3.4 at least, e.g.

.. doctest::
   :pyversion: > 3.3

[tkem]

Using Sphinx doctest, we may also use something like

>>> datetime.date.now()   # doctest: +SKIP
datetime.date(2008, 1, 1)

e.g. to get away with timings, etc.