Made PasswordResetForm compatible with Django 1.6

Django 1.6 changed the way unusable passwords work. Part of this included the removal of the UNUSABLE_PASSWORD variable. This was handled by importing UNUSABLE_PASSWORD_PREFIX when UNUSABLE_PASSWORD was unavailable. The problem is that PasswordResetForm's clean_email method still contains a reference to UNUSABLE_PASSWORD, which would be undefined when using Django 1.6, leading to a NameError on form validation.

My solution uses the is_password_usable function, which is defined in both Django 1.5 and 1.6 (and earlier). I've written tests to verify that PasswordResetForm correctly handles usable and unusable passwords.