optional bias regularizer

Sometimes, bias weights are ignored from regularization as mentioned in UFLDL tutorials at http://ufldl.stanford.edu/wiki/index.php/Backpropagation_Algorithm. nn.biasPenaltyL2 is a boolean field that determines whether or not to use reguarlization on the bias weights. nnsetup() initializes this term to be true by default (which is the current configuration of the Toolbox) so this change will be backward compatible with any codes that uses this Toolbox