Add gpu_util.cuh, with caffe_gpu_atomic_add
Add gpu_util.cuh, with caffe_gpu_atomic_add

(double impl from NVIDIA dev docs; float impl included in CUDA as
"atomicAdd")

tackgeun/caffe,longjon/caffe,longjon/caffe,gnina/gnina,tackgeun/caffe,CZCV/s-dilation-caffe,CZCV/s-dilation-caffe,gnina/gnina,gnina/gnina,tackgeun/caffe,longjon/caffe,wangg12/caffe,CZCV/s-dilation-caffe,wangg12/caffe,CZCV/s-dilation-caffe,longjon/caffe,gnina/gnina,tackgeun/caffe,wangg12/caffe,gnina/gnina,gnina/gnina,wangg12/caffe