Add parsing of "initialize" qobj instruction

[chriseclectic]

Summary

Adds parsing of "initialize" qobj instruction for Issue #82

Details and comments

This parses a qobj instruction of the form:

{"name": "initialize", "qubits": qs, "params": psi}

where qs is a list of qubits eg [a, b, c], and psi is a (JSON) complex vector corresponding to tensor product order psi[c] \otimes psi[b] \otimes psi[a]

[atilag]

LGTM,
I'd think about having just one method for validation, not quite sure if we need this granularity elsewhere thought