Support for Metadata in SparkPost Django Email Backend

[kingb]

Turns out the SparkPost Django Email Backend did not support sending metadata via the transmission API. I've added support for sending email with metadata and added documentation to docs/django/backend.rst. I've also created some example code in examples/django/send_email_with_metadata.py

Implementation of issue #175.

[hugoseabra]

Hey guys. There is a long time this is to be reviewed. So simple and so important.

Let's keep moving with this and merge it.