Adding Resource Owner for Paypal

Adds a new `ResourceOwner` for [PayPal](http://paypal.com) following OAuh2 integration from [Log In with PayPal](https://developer.paypal.com/webapps/developer/docs/integration/direct/identity/log-in-with-paypal/).
Why `preg_replace()`? `str_replace()` or `strtr()` would do the job.