Refactor, rename Payment#composePayRequestData to Payment#composePayRequest
Update the comments on Paypal#_remoteCall
Store the preapproval key to db in POST /paypal/preapproval
Store the pay key to db, when POST /paypal/pay
Let Paypal#verify return a boolean result in callback
Add new test helper methods, database.dropModels & database.unloadFixtures
To update payment info when receiving preapproval IPN from payapl