Change to use tox to manage unit tests

tox.ini

@@ -0,0 +1,18 @@
+[tox]
+skipsdist = True
+usedevelop = True
+envlist = py{34,35,36,37}-dj{111,20}
+
+[testenv]
+setenv =
+    DJANGO_SETTINGS_MODULE = settings
+basepython =
+    py34: python3.4
+    py35: python3.5
+    py36: python3.6
+    py37: python3.7
+deps=
+    dj111: Django>=1.11.1,<2.0
+    dj20: Django>=2.0a1
+install_command = pip install -e ".[testing]" -U {opts} {packages}
+commands=coverage run runtests.py -v 2