Add tox.ini to test and collect combined coverage data of multiple Python and Django versions

tox.ini

@@ -0,0 +1,36 @@
+[tox]
+envlist = django{1.8,1.9,1.10,1.11}-{py27,py34,py35},coverage
+
+[testenv]
+deps =
+    coverage
+    django1.8: Django>=1.8,<1.9
+    django1.9: Django>=1.9,<1.10
+    django1.10: Django>=1.10,<1.11
+    django1.11: Django>=1.11a1,<1.12
+whitelist_externals = cd
+setenv =
+    PYTHONPATH = {toxinidir}/sample_project
+    PYTHONWARNINGS = module
+    PYTHONDONTWRITEBYTECODE = 1
+commands = 
+    coverage run -p sample_project/manage.py test app
+
+[testenv:coverage]
+deps = coverage
+skip_install = true
+commands =
+    coverage combine
+    coverage report
+    coverage html
+
+[coverage:run]
+branch = True
+parallel = True
+source =
+    adminsortable
+    sample_project
+
+[coverage:report]
+exclude_lines =
+    if __name__ == .__main__.: