First working version.

 * Add offset parameter to add_block_sums().
 * Add a new your_sort() that calls radix_sort().
 * In the main loop in radix_sort(), iterate over all 32 bit positions.
 * With the new parameter "negate", scatter() now works on both zeros and ones.
 * Use cudaMemcpy() to write the sorted data into the right array.
Remove d_histogram and make_histogram().
Use MAX_BIT.
Calculate the grid_size. Rename block_size to block_dim.
Remove a hard-coded value.
Remove three unnecessary cudaMemset().