Merge pull request #1432 from Yangqing/dev

fix relu cudnn test bug
in SoftmaxLossLayer, check label >= 0 in addition to upper bound
use DCHECK in SoftmaxLossLayer so that bounds checking has no perf penalty without DEBUG
Merge pull request #1469 from longjon/label-lower-bound

Check both lower and upper bounds in SoftmaxLossLayer