* 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
* Add smtp-id in unique_args (metadata), to ensure
it shows up in click and open events.
* Add SENDGRID_GENERATE_MESSAGE_ID setting,
default True, to control auto-Message-ID behavior.
* Document it.
Simplify install to just `pip install django-anymail`.
(Rather than `... django-anymail[mailgun]`
All of the ESPs so far require requests, so just move
that into the base requirements. (Chances are your
Django app already needs requests for some other
reason, anyway.)
Truly unique ESP dependencies (e.g., boto for
AWS-SES) could still use the setup extra features
mechanism.
