Use the newly implemented caffe_gpu_{add,sub} in the GPU device wrapper
Replace caffe_gpu_{copy+axpy} with sub in SigmoidCrossEntropyLossLayer
Unify the CPU/GPU Forward/Backward of the SigmoidCrossEntropyLossLayer
Merge the CPU/GPU Forward/Backward of the SoftmaxWithLossLayer
Use {const, mutable}_{data, diff} in the unified Forward/Backward
Unify the CPU/GPU Forward/Backward of the InnerProductLayer
Unify the CPU/GPU Forward/Backward of the SplitLayer
Unify the CPU/GPU Forward/Backward of the EltwiseLayer
Add im2col and col2im to wrap im2col_{cpu, gpu} and col2im_{cpu, gpu}
Unify the CPU/GPU versions of Forward/Backward of the ConvolutionLayer