Return the id and shift of the code with the minimum bit error

[meyerj]

…from Decoder::decoder().

We did observe some spurious detections of the wrong code, if multiple candidates are sufficiently close, depending on the value of errorCorrection passed to the STag constructor. Even for high values it may be desirable to return the candidate with the least amount of bit errors.

We are using this patch for a while now in our fork (Intermodalics/stag_ros#1), and the number of false detections has been reduced. What do you think?