Probabilities snapshot extension

[chriseclectic]

What is the expected behavior?

This will be used by various methods of the QasmSimulator to return snapshots of the measurement outcome probabilities of the specified qubits.

class SnapshotProbabilities(Snapshot):
    __init__(self, label, num_qubits, variance=False):
        if variance:
            super(label, "probabilities_with_variance", num_qubits, 0, None)
        else:
            super(label, "probabilities", num_qubits, 0, None)


# Monkey patch function
def snapshot_probabilities(self, label, qubits=None, variance=False):
     # If qubits is None snapshot all qubits.

circuit.snapshot_probabilities = snapshot_probabilities