CUDNN Julia port

I just finished implementing CUDNN.jl, a Julia wrapper for the NVIDIA cuDNN GPU accelerated deep learning library. It consists of a low level interface and a high level interface. The low level interface wraps each function from libcudnn.so in a Julia function in libcudnn.jl and each data type from cudnn.h in a Julia datatype in types.jl. These were generated semi-automatically using Clang and are documented in the cuDNN Library User Guide. For the high level interface defined in CUDNN.jl, I kept the original names from the C library and provided more convenient type signatures, return values, and keyword arguments with reasonable defaults.  Next step will be integrating this with Knet.jl.