Added support for setting custom django permissions in content types

Currently FeinCMS ignores custom permissions which are set in content type
models.
So, for example, this code will not work as expected (defined permissions are
ignored):

    class SomeCustomContent(models.Model):
        ...
        class Meta:
            abstract = True
            permissions = (
                ("view_task", "Can see available tasks"),
                ("change_task_status", "Can change the status of tasks"),
                ("close_task", "Can remove a task"),
            )


This change adds support for custom permissions in content type models.