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/).
@stof fourth parameter of `str_replace` must be a reference, since it would be set to the number of replacements made, not used as a limit, so that's why I choose `preg_replace` over `str_replace`.