Initial stab at addressing qutip/qutip#88.

This is untested so far, but is mainly to demonstrate a
proposed design approach for addressing superoperator metadata,
and how this might be used to make more robust and useful
superoperator conversion functions.
Fixed silly mistakes in previous commit.
Added iscptp, to_super and support for printing choi, super.
Added unit tests for new metadata.
Fixed missing type= in kraus conversion.
Refactored type='super' and type='choi' in line w/ #88.

New attribute uses superrep='super' and superrep='choi' instead.
Added to_kraus, average_gate_fidelity as example of new superrep attr.
Addressed comments by @jrjohansson on PR #90.
Added test of average_gate_fidelity.
Fixed Py3k compatibility, slow computation in __str__ and _repr_latex_.
Merge pull request #90 from cgranade/feature-supermeta

Implementation of type="super" and type="choi" distinction.