Add tox to test different python and django versions

tox.ini

@@ -0,0 +1,24 @@
+[tox]
+envlist =
+    py{27,34,35}-django19
+    py{27,34,35}-django110
+    py{27,34,35,36}-django111
+    py{34,35,36}-django20
+    lint
+
+[testenv]
+commands = py.test {posargs}
+extras = test
+pip_pre = true
+deps =
+    django19: django>=1.9,<1.10
+    django110: django>=1.10,<1.11
+    django111: django>=1.11,<2.0
+    django20: django>=2.0
+
+[testenv:lint]
+basepython = python3.6
+deps =
+    flake8
+commands =
+    flake8 django_migration_linter