Added support for `scope` query parameter

As outlined by http://tools.ietf.org/html/rfc6749#section-3.3

> The authorization and token endpoints allow the client to specify the scope of the access request using the "scope" request parameter."

This adds support for a `scope` query parameter while keeping backwards compatibility for the original `scopes` parameter.

See https://github.com/evonove/django-oauth-toolkit/issues/100 for original issue.