Add relations
Test has light authenication
Add authenication tests for legal users
Add Mangopay Wallet and wallet tests
Add wallet task
Fix Strong authentication types
Merge pull request #5 from FundedByMe/better-relations

Improvements (Sorry its a bunch of stuff in one PR)
Add MangoPay Card model
Get Card ID from the CardRegistration object
Make MangoPay Card field mullable and one-to-one relationship
Add tests for MangoPay Cards and CardRegistration