Support for cc/bcc in Django backend

[richleland]

• Refactored to use a custom message class that just preps data for the API (SparkPostMessage)
• Added test utils file for the at_least_version function
• Was able to really simplify the backend code - most of the logic lives in the message class now

[ewandennis]

Django's EmailMessage wants a list for the reply_to field but we expect a string. A naive solution might be to concat the list with semicolons:

formatted['reply_to'] = ';'.join(message.reply_to)

Without this, using EmailMessage(reply_to=[...]) causes an API error:

SparkPostAPIException: Call to https://api.sparkpost.com/api/v1/transmissions returned 422, errors:

        invalid data format/type: field 'content.reply_to' is of type 'json_array', but needs to be of type 'string'

[ewandennis]

Tested successfully on Django 1.9.4, python 2.7.11.
Test suite succeeded on python 2.7.11 and 3.5.1.

We no longer throw UnsupportedParam so people can remove except clauses. Worth noting in changelog?

We might want to include example code showing how to use CC etc with Django's EmailMessage? Something like this:

EmailMessage(
  subject='Hello from Django and python-sparkpost',
  body='Hello Django monsters!',
  from_email='django@sparkpostbox.com',
  to=['someone@somedomain.com'], 
  cc=['another@somedomain.com'],
  bcc=['athird@somedomain.com'],
  reply_to=['yetanother@somedomain.com']
).send()

[richleland]

Thanks for the thorough review @ewandennis - can you take a look at the updates?