* Update utils.get_anymail_setting to support
kwargs override of django.conf.settings values
* Use the updated version everywhere
* Switch from ImproperlyConfigured to
AnymailConfigurationError exception
(anticipates feature_wehooks change)
Closes#12
Treat Mailgun metadata like all other ESPs: simple
key-value dict, where values are strings. If you want
to store JSON in metadata, you should serialize and
deserialize it yourself.
