Merge pull request #586 from sharetribe/generalize-entity-utils

move EntityUtils out of MarketplaceService module
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