Fixed breaking changes from saction 0.4, and removed deprecated calls to sanction.client

Sanction 0.4 introduced some breaking changes. If downloading sanction using `easy_install`, sanction 0.4 is selected as the version to install. This broke django-sanction and resulted in OAuth2 simply not working.

This change fixes the calls to `sanction.Client.request_token` and `sanction.Client.auth_uri` and `sanction.Client.__init__`. 

Additionally, the `Client` class is imported directly from the `sanction` module, to avoid deprecation warnings.