add cuDNN to build
call __signbit for CUDA >= 6.5 implementation
strategize cuDNN convolution
strategize cuDNN pooling
strategize cuDNN activations: ReLU, Sigmoid, TanH
strategize cuDNN softmax
CUDNN_CHECK
report cuDNN error string
[docs] include cuDNN in installation and performance reference
Merge pull request #1046 from shelhamer/cudnn

cuDNN acceleration