Add operator-ket and operator-bra support to super_tensor.

See #238 for more details, but this extends ``_permute`` and ``super_tensor`` to act correctly on ``operator-ket`` as well as ``super, superrep="super"``.
I think we should use the Qobj properties `is_bra`, `is_oper`, etc. instead of directly comparing the type property with a string. This might be a good time to change this.