Added UserID aka PayerID scenario. Request objects are now slightly diff...

...erent to accomodate for multiple kinds of `receiverTypes`. Running tests are now dependent on `test-values.js` instead of environment variables. Updated vows and underscore dependencies in `package.json`.

Obtaining the `UserID` or `PayerID` is made possible via the PayPal Classic API. The best case scenario would be to use the PayPal Account Authentication Service (aka Login with PayPal). It is also possible to obtain a customer's `PayerID` via Express Checkout.

**Links:**
https://developer.paypal.com/docs/classic/account-auth-service/gs_account_auth_service/
https://developer.paypal.com/docs/classic/paypal-payments-pro/integration-guide/WPECIntegration/