QubitPermutationGate should be json serializable and define a _decompose_

[mpharrigan]

Right now, QubitPermutationGate cannot be resolved during deserialization. I don't think there's any big reason why it isn't. In fact: I would posit that it's really important that all gates be serializable.

[tanujkhattar]

It should also define a _decompose_ in terms of swaps.