Updates gem to use latest endpoints and updates documentation

* Updates Gem to use new endpoints
* Updates Gem to fetch all available attributes
* Updates Gem to uses a slightly wider default scope so it returns the `user_id`
* Updates README to include links to new documentation
* Updates README to list all the current scopes and attributes
* Updates README to reflect new name (Log In With PayPal vs PayPal Access)
* Updates README with a link to my tutorial on how to use this Gem