SendGrid does not always correctly provide the sent Message-ID header value
to a tracking webhook's smtp-id field, making it unreliable to use for Anymail's
`message_id`.
Instead, generate a UUID `message_id` for Anymail tracking, and pass it from
send to webhooks in SendGrid custom args as anymail_id.
Webhooks will fall back to smtp-id for compatibility with previously-sent
messages that didn't have an anymail_id custom arg.
Fixes#108
* 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
