Initial support for CuDNN R2 (tested only on OSX)

There was a new CuDNN release (R2 RC1) with OSX support. These are the changes I made to get the GPU backend working on OSX, let me know what you think and you if want me to submit a PR against dev etc...

N.b. This code requires CuDNN R2, and is incompatible with R1.

P.S. the new CuDNN release supports padded pooling (but I haven't made modifications to support it as yet).
P.P.S. Fantastic work on this lib : )