add cuDNN 4 support

[MichaelHirn]

This issue tracks full cuDNN4 support with rust-cudnn.

cuDNN 4 does not introduce many new things and rust-cudnn works already pretty well with the cuDNN 4 library. (see notable update below) Mostly it makes improvements to the Normalization API and internally improves the convolution performance. From the cuDNN 4 release notes.

_New Features_
- Batch Normalization routines have been added.
- Convolution forward and backward now supports NHWC tensor format.
- FFT Tiling algorithm has been added for cudnnConvolutionForward and cudnnConvolutionBackwardData routines
- cudnnConvolutionForward now supports computation in FP16 when run on GPU
with a compute capability >= 5.3
- cudnnConvolutionForward has been optimized for batch size = 1
- Pooling and activation routines have a descriptor option to propagate NaN numbers.

One notable update of cuDNN 4 is

Performance of cudnnConvolutionBackwardFilter when using Algo 1 has been
improved for some cases. This code path now also requires a workspace.

which affects collenchyma-nn as it makes the convolution algorithm inconsistent when switching from cuDNN3 to cuDNN4.

[hobofan]

#21 adds "passive" cuDNNv4 support, meaning that it knows about the FFT Tiling algorithm, so it won't break when cuDNNv4 is used, but doesn't implement any of the other new features.

[jramapuram]

Would be nice to have forward/backward pass for LSTM's.