Add scenarios to customers, orders, and credits

Added the following scenarios: 
https://github.com/balanced/balanced-api/blob/revision1/scenarios/orders/basic_usage.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/orders/cannot_over_credit.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/orders/create.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/orders/create_refund.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/orders/failed_refund_pending.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/orders/transaction_inherit_description.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/orders/unverified_merchant.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/customers/set_default_destination.yml
https://github.com/balanced/balanced-api/blob/revision1/scenarios/customers/underwrite.yml