Merge of fixed_qcircuit_measurement branch to allow simple measurements. 

The changes in this PR make a "fake" measurement operators Mx and
Mz. These does not implement real quantum measurement, is specially
flagged with a "measurement" property so that the circuitplot routines
can identify the qubits after this operation as having been
measured. The circuitplot utilities then double the qubit wires after
this gate, as well as the controlled lines hanging off the qubit
wires.

This is a fixed version of an earlier PR attempt that was closed after
it started looking grotty after I mistakenly rebased.