Add support for subsampling and gradient to GpuDnnConv

This adds a grad() method to GpuDnnConv and adds support for subsampling.  It also allows choosing between convolution and cross-correlation mode (however cross-correlation mode has not been tested).

I've also made a base class that create the cudnn handle to make other ops easier to write.
+1 for cleaning this up! Maybe some comments would be good... `# ensure correct op is in compiled graph`, `# ensure perfect reproducibility of gpu results`, `# ensure gpu results are close to cpu results`