Merge pull request #4 from FundedByMe/documents

Documents
Merge pull request #5 from FundedByMe/better-relations

Improvements (Sorry its a bunch of stuff in one PR)
Merge pull request #7 from FundedByMe/payout

Payout
Add MangoPay Card model
Get Card ID from the CardRegistration object
Make MangoPay Card field nullable and one-to-one relationship
Add tests for MangoPay Cards and CardRegistration
Fix MangoPay User ID when creating CardRegistration
Better wording and assertions
Merge pull request #6 from FundedByMe/cards

Cards
Switch usernam
Add payout task
Spell currency correctly
Currency should be a str
Fix python_money
Fix python_money more
To int
Add payout tests
Finish Payout tests