Multi label Data and MultiLabel Accuracy

This PR allows to use multi-label data {-1,0,1} with 0 means ignore, -1 negative and 1 positive.

@sergeyk review
Thank @jeffdonahue I will restore `SigmoidCrossEntropyLayer` and create a `MultiLabelLoss` which follows the logic of 0 means ignore, -1 negative and +1 positive.