Custom related_names

This release contains a backward incompatible change:

• Foreign key reverse names have been specified, so this library can be
  installed alongside django-oauth-toolkit. Code that traverses from
  the User model to django-oauth2-provider models will need to update the
  related name used.

  >>> user.access_token
  >>> user.grant
  >>> user.refresh_token
  

  becomes:

  >>> user.dop_access_token
  >>> user.dop_grant
  >>> user.dop_refresh_token