Add sort_by_key for oclMat.
Most codes are ported from AMD's Bolt library.
Four methods are implemented:

SORT_BITONIC,   // only support power-of-2 buffer size
SORT_SELECTION, // cannot sort duplicate keys
SORT_MERGE,
SORT_RADIX      // only support signed int/float keys
Fix build errors
Some modifications to sortByKey API.
Add documentation.
Fix doc build error.
Merge pull request #1189 from pengx17:2.4_sort_by_key