Tests: Simplify Django version compatibility

* Restructure runtests.py as suggested in
  https://docs.djangoproject.com/en/1.9/topics/testing/advanced/#using-the-django-test-runner-to-test-reusable-applications

* Load version-specific Django settings modules
  (rather than trying to make a single settings.configure()
  work with all supported versions).

* Use `django-admin startproject` defaults for all
  settings modules. (Tests compatibility with typical
  apps, middleware, and other settings.)

* Set up tests-specific url config; switch to literal
  urls in test cases. (Eliminates url `reverse` from
  tests.)

* Make runtests.py executable

Closes #18

tests/test_settings/urls.py

@@ -0,0 +1,5 @@
+from django.conf.urls import include, url
+
+urlpatterns = [
+    url(r'^anymail/', include('anymail.urls')),
+]