Add method to simplify generating a token via Authorizations API

* Returns the existing Authorization if one has already been created
  for this user for this application.
* Creates a new authorization if one does not exist.
Add methods to help OAuth Application Owners manage authorizations

* New method allows OAuth application owners to revoke all
  authorizations to their application.
* New method allows OAuth application owners to revoke a single
  authorization token to their application.