Added LowCreditNotification model.
Added manager for LowCreditNotification model
Added migration for LowCreditNotification model.
Added tests for LowCreditNotification model.
Added sending mail to billing notification model.
Added template for low credits notification mail.
Added tests for sending mail for low credit notification.
Create new notification from within the billing API
Merge branch 'develop' into feature/issue-878-implement-low-credit-alerts-in-billing
Added temporary testing things for sending low credit notification email
Added confirm sent after email successfully sent
Remove temporary billing test script.
pep8 cleanup
Change email text to what was discussed: https://gist.github.com/SteveBarnett/6c5929ab1efd3c7eafdd
Fix failing test
Merge branch 'develop' into feature/issue-878-implement-low-credit-alerts-in-billing

Conflicts:
	go/billing/models.py
Moved creating low credit notification to task to be called from API
Updated tests to use new celery task
Changed to using settings for billing email
Renamed migration for numerical order