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/settings_1_8.py

@@ -0,0 +1,103 @@
+"""
+Django settings for Anymail tests.
+
+Generated by 'django-admin startproject' using Django 1.8.
+
+For more information on this file, see
+https://docs.djangoproject.com/en/1.8/topics/settings/
+
+For the full list of settings and their values, see
+https://docs.djangoproject.com/en/1.8/ref/settings/
+"""
+
+# Build paths inside the project like this: os.path.join(BASE_DIR, ...)
+import os
+
+BASE_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+
+
+# Quick-start development settings - unsuitable for production
+# See https://docs.djangoproject.com/en/1.8/howto/deployment/checklist/
+
+# SECURITY WARNING: keep the secret key used in production secret!
+SECRET_KEY = 'NOT_FOR_PRODUCTION_USE'
+
+# SECURITY WARNING: don't run with debug turned on in production!
+DEBUG = True
+
+ALLOWED_HOSTS = []
+
+
+# Application definition
+
+INSTALLED_APPS = (
+    'django.contrib.admin',
+    'django.contrib.auth',
+    'django.contrib.contenttypes',
+    'django.contrib.sessions',
+    'django.contrib.messages',
+    'django.contrib.staticfiles',
+    'anymail',
+)
+
+MIDDLEWARE_CLASSES = (
+    'django.contrib.sessions.middleware.SessionMiddleware',
+    'django.middleware.common.CommonMiddleware',
+    'django.middleware.csrf.CsrfViewMiddleware',
+    'django.contrib.auth.middleware.AuthenticationMiddleware',
+    'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
+    'django.contrib.messages.middleware.MessageMiddleware',
+    'django.middleware.clickjacking.XFrameOptionsMiddleware',
+    'django.middleware.security.SecurityMiddleware',
+)
+
+ROOT_URLCONF = 'tests.test_settings.urls'
+
+TEMPLATES = [
+    {
+        'BACKEND': 'django.template.backends.django.DjangoTemplates',
+        'DIRS': [],
+        'APP_DIRS': True,
+        'OPTIONS': {
+            'context_processors': [
+                'django.template.context_processors.debug',
+                'django.template.context_processors.request',
+                'django.contrib.auth.context_processors.auth',
+                'django.contrib.messages.context_processors.messages',
+            ],
+        },
+    },
+]
+
+WSGI_APPLICATION = 'tests.wsgi.application'
+
+
+# Database
+# https://docs.djangoproject.com/en/1.8/ref/settings/#databases
+
+DATABASES = {
+    'default': {
+        'ENGINE': 'django.db.backends.sqlite3',
+        'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
+    }
+}
+
+
+# Internationalization
+# https://docs.djangoproject.com/en/1.8/topics/i18n/
+
+LANGUAGE_CODE = 'en-us'
+
+TIME_ZONE = 'UTC'
+
+USE_I18N = True
+
+USE_L10N = True
+
+USE_TZ = True
+
+
+# Static files (CSS, JavaScript, Images)
+# https://docs.djangoproject.com/en/1.8/howto/static-files/
+
+STATIC_URL = '/static/'