Django 1.8's reset_warning_registry (which we backport)
was generating `RuntimeError: dictionary changed size
during iteration` in Travis tests under Python 3.x.
Likely a thread-safety issue on sys.modules.values().
See https://code.djangoproject.com/ticket/21049
for fix applied elsewhere in Django.
* Add some context to exceptions on unserializable
values (addresses #89).
* Document need to format merge data
(into something JSON-serializable).
* Add RemovedInDjrill2 DeprecationWarning.
* Deprecate blanket date/datetime serialization.
Second attempt to address possible test sequencing issue around tests that alter settings. (Failures in Travis tests not reproducible locally.)
Back-ports override_settings from Django 1.4 for compatibility with Django 1.3.
