add tox config for testing against multiple Python versions

Author: Sara Safavi

tox.ini

@@ -0,0 +1,13 @@
+# Tox (https://tox.readthedocs.io/) is a tool for running tests
+# in multiple virtualenvs. This configuration file will run the
+# test suite on all supported python versions. To use it, "pip install tox"
+# and then run "tox" from this directory.
+
+[tox]
+envlist = py27, py34, py35, py36
+
+[testenv]
+deps = pytest
+commands = 
+    pip install -e .[dev]
+    pytest