Add post_save hook for creating billing accounts when UserProfiles and Riak accounts are created.
Explicitly raise an error with a useful message when an account cannot be retrieved while creating a billing transaction.
Add management command for creating billing accounts.
Fix billing account creation tests to take into account that user creation hooks create billing accounts.
Print note about user accounts that already had a billing account.
Add test for post-save hook that creates a billing account when a new Django user account is created.
Add tests for post save hook that creates billing accounts.
Merge branch 'feature/issue-888-create-billing-accounts' into develop