Entity builder
Merge pull request #587 from sharetribe/entity-builder

Entity builder
Boilerplate for PaypalService::Merchant
setup billing agreement api call
Create billing agreement api call
Hide address fields when requesting bill agr
Remodel billing agr, model + migration
add username_to to all merchant responses
Add billing agreement data to paypal account entity
Scaffolding for billing agreement actions
DoReferenceTransaction API operation, WIP

- extract api building to allow setting subject per request