Reject inactive user password recovery

If you try to reset the password from an inactive user, you'll be able to do it, but you'll not be able to login anyway.
Therefore I created a check inside the form "clean_user" field. If the user is inactive and the app settings `RECOVER_ONLY_ACTIVE_USERS=True`, then an error message will be displayed saying that an inactive user can't recover his password.

By default `RECOVER_ONLY_ACTIVE_USERS` is set to `False` in order to make this change backwards compatible.

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Can you use `with self.settings(…)` instead? It's a bit less typing an more elegant.

See https://docs.djangoproject.com/en/1.4/topics/testing/#django.test.TestCase.settings