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/__init__.py

@@ -0,0 +1,14 @@
+# These are the default "django-admin startproject" settings.py files
+# for each supported version of Django, with:
+#
+#   * "anymail" appended to INSTALLED_APPS
+#   * ROOT_URLCONF = 'tests.test_settings.urls'
+#
+# Keeping a file for each Django version is simpler than trying
+# to maintain warning-free compatibility with all Django versions
+# in a single settings list.
+#
+# It also helps ensure compatibility with default apps, middleware, etc.
+#
+# You can typically find the default settings.py template in
+# SITE_PACKAGES/django/conf/project_template/project_name/settings.py-tpl