HW4: Switched completely to the GPU Gems radix sort

http://http.developer.nvidia.com/GPUGems3/gpugems3_ch39.html

Finally got it all parallelized. However, online performance is around
20 ms, but should actually be more in the vicinity of 5 ms or so. Needs
some performance improvements.

Limitations: Image cannot have more than 1,024 x 1,024 = 1,048,576 pixel
(1 Megapixel), provided the maximum number of threads per block is
1,024. Otherwise it needs to be lowered accordingly.