MedianFilter OpenCL kernels optimization

Added new Intel-optimized 3x3 and 5x5 kernels to medianFilter.cl file and corresponding code to host in smooth.cpp
Only Intel platform and 1 channel images are supported.

build_examples=OFF
check_regression=*Median*

http://ocl.itseez.com/intel/export/perf/pr/2794/report/