Update library to 2.1.0.

* Add support for Multiple Items per payment.
* Add `clearAllUserData` method to `PayPalMobile`.
* If app provides `defaultUserEmail`, `defaultUserPhoneCountryCode`, and/or `defaultUserPhoneNumber`,
  these will now be overridden by the user's most recent PayPal login.
* Update PayPal logo.
* Update card.io library to 3.7.0.