Don't use PYTHON_VERSION_STRING when searching for Python libraries

It may be an arbitrary string, and, for example, in Ubuntu 13.10
it's set to "2.7.5+", which breaks the find script.
added mask support to cv::norm, cv::meanStdDev
added some performance tests
added cv::PSNR performance test
added NORM_L2SQR type to cv::norm
changed code in python tutorial, these functions don’t return image
Merge pull request #2121 from ilya-lavrenov:tapi_perf
Merge pull request #2138 from alekcac:python_tutorial_fix
Merge pull request #2136 from SpecLad:python-ver