URLField.gen_value doesn't return especially unique values

[Ichimonji10]

Method URLField.gen_value frequently generates identical values. As a result of this issue, several work-arounds have been added to module nailgun.entities. One example is at Media.create_missing:

    def create_missing(self):
        """Give the ``path_`` instance attribute a value if it is unset.

        By default, :meth:`nailgun.entity_fields.URLField.gen_value` does not
        return especially unique values. This is problematic, as all media must
        have a unique path.

        """
        if not hasattr(self, 'path_'):
            self.path_ = gen_url(subdomain=gen_alpha())
        return super(Media, self).create_missing()

Let's fix gen_url and drop the work-arounds from module nailgun.entities.

[Ichimonji10]

#190 fixes this.