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
Add IPP mean and sum functions
Fix build errors
Some changes in sum and mean functions
Some modifications to sortByKey API.
Add documentation.
Fix doc build error.
Update OpenCVDetectPython.cmake

On my MacOS the output of the sphinx-build binary is the following:

Error: Insufficient arguments.

Sphinx v1.2b1
Usage: /usr/local/bin/sphinx-build [options] sourcedir outdir [filenames...]

...

I have installed sphinx, but without this change HAVE_SPHINX==0 for me, so I can't build docs.
Removed useless .*
Merge pull request #1216 from alekcac:sphinx
Make ts always static.

This allows us to forget about the GTEST_(CREATE|LINKED_AS)_SHARED_LIBRARY
macros and to get rid of the dependency on cvconfig.h.
Merge pull request #1231 from SpecLad:static-ts
Merge pull request #1187 from kdrobnyh:mean_and_sum
Merge pull request #1189 from pengx17:2.4_sort_by_key