add tox to project

tox.ini

+# this file is *not* meant to cover or endorse the use of tox or pytest or
+# testing in general,
+#
+#  It's meant to show the use of:
+#
+#  - check-manifest
+#     confirm items checked into vcs are in your sdist
+#  - python setup.py check (using the readme_renderer extension)
+#     confirms your long_description will render correctly on pypi
+#
+#  and also to help confirm pull requests to this project.
+
+[tox]
+envlist = py{27,34,35,36}
+
+[testenv]
+basepython =
+    py27: python2.7
+    py34: python3.4
+    py35: python3.5
+    py36: python3.6
+deps =
+    check-manifest
+    readme_renderer
+    flake8
+    pytest
+commands =
+    check-manifest --ignore tox.ini,tests*
+    python setup.py check -m -r -s
+    flake8 .
+    py.test tests
+[flake8]
+exclude = .tox,*.egg,build,data
+select = E,W,F