Add scenarios to customers, orders, and credits

[rserna2010]

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

[steveklabnik]

This looks good, thanks!

I'm going to leave this here, and pull them in as I make them actually work. Feel free to keep adding commits here.

[steveklabnik]

Pulled this in, thanks!