Add tox to test multiple python environments.

tox.ini

@@ -0,0 +1,17 @@
+# Tox (http://tox.testrun.org/) 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 = py34, py35
+
+[testenv]
+passenv = TRAVIS TRAVIS_JOB_ID TRAVIS_BRANCH
+deps =
+    coverage
+    unittest2
+commands =
+    coverage erase
+    coverage run {toxinidir}/tests/run_tests.py
+    coverage report --include={toxinidir}/*githubcli/*