Fixed warper selection bug in stitching_detailed. Removed estimation of aspect ratio in BA in stitching to avoid stretching of input images. Did minor refactoring.

Updated bundle adjustment in stitching module: 1) it minimizes reprojection error now, 2) it minimizes error over focal, aspect, p.p.x, p.p.y parameters. Refactored and updated warpers.

updated feature_homography.py sample to use new features

changed OpenCV version to 2.3.2

wrapped StarFeatureDetector and GridAdaptedFeatureDetector

Since there is no a standard functions cv::min and cv::max which has cv::Scalar argument, removed the performance tests, 
calling the functions cv::min and cv::max with cv::Scalar argument.
Instead of them two performance tests were created which call the functions cv::min and cv::max with 'double' argument.