Move tests out of app module

(Directory structure as suggested in
[Django testing docs][1].)

[1]: https://docs.djangoproject.com/en/1.9/topics/testing/advanced/#using-the-django-test-runner-to-test-reusable-applications

MANIFEST.in

@@ -1,3 +1,2 @@
 include README.rst AUTHORS.txt LICENSE
 recursive-include anymail *.py
-prune anymail/tests

docs/contributing.rst

@@ -78,6 +78,6 @@ get from from environment variables. Look in the ``*_integration_tests.py``
 files in the `tests source`_ for specific requirements.
 
 .. _.travis.yml: https://github.com/anymail/django-anymail/blob/master/.travis.yml
-.. _tests source: https://github.com/anymail/django-anymail/blob/master/anymail/tests
+.. _tests source: https://github.com/anymail/django-anymail/blob/master/tests
 .. _mock: http://www.voidspace.org.uk/python/mock/index.html
 .. _tested on Travis: https://travis-ci.org/anymail/django-anymail

runtests.py

@@ -52,7 +52,7 @@ except ImportError:
 
 def runtests(*args):
     test_runner = TestRunner(verbosity=1)
-    test_labels = args if len(args) > 0 else [APP]
+    test_labels = args if len(args) > 0 else ['tests']
     failures = test_runner.run_tests(test_labels)
     sys.exit(failures)