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.

added first version of public stitching API, added simple sample which uses that API, old sample renamed to stitching_detailed

added gpu::buildWarpPlaneMaps and gpu::buildWarpCylindricalMaps functions, integrated into stitching

GPU: updated upsample, downsample functions, added pyrDown, pyrUp, added support of 16S filtering; put spherical warper on GPU (from opencv_stitching)

fixed bug in opencv_stitching (corrected resize images step), added matches checking (both 1->2 and 2->1 must be presented)

fixed error in gcgraph which led to problems in opencv_stitching GC seam finder, added check for duplication in matches (opencv_stitching)