Use tox for running tests and building docs

* Set up tox for testing supported Django/Python combinations
* Also include tox env for checking and building docs
* Use tox-travis for Travis CI integration
* Add tests against Django master
* Document building docs and running tests with tox

docs/_readme/docutils.cfg

@@ -0,0 +1,31 @@
+# docutils (rst2html) config for generating static HTML that approximates
+# PyPI package description rendering (as of 3/2018).
+#
+# Usage (in package root dir):
+#   python setup.py --long-description | rst2html.py --config=docs/_readme/docutils.cfg > ${OUTDIR}/readme.html
+#
+# Requires docutils and pygments (both are installed with Sphinx)
+
+[general]
+# Duplicate docutils config used by PyPA readme_renderer.
+# https://github.com/pypa/readme_renderer/blob/master/readme_renderer/rst.py
+# (This isn't exactly what's used on legacy PyPI, but it's close enough.)
+cloak_email_addresses = True
+doctitle_xform = True
+sectsubtitle_xform = True
+initial_header_level = 2
+file_insertion_enabled = False
+math_output = MathJax
+raw_enabled = False
+smart_quotes = True
+strip_comments = True
+syntax_highlight = short
+
+# Halt rendering and throw an exception if there was any errors or warnings from docutils.
+halt_level = 2
+# DON'T Disable all system messages from being reported.
+#       (We're not running inside readme_renderer, so *do* want to see warnings and errors.)
+# report_level = 5
+
+# Approximate PyPI's styles:
+stylesheet = docs/_readme/readme.css