seeds: add SEEDS superpixels implementation to ximgproc module
seeds: add documentation
seeds: add C++ and python samples
add gitignore (from opencv main repository)
seeds sample: fix compiler warnings

the exact warnings are:
/builds/precommit-contrib_macosx/opencv_contrib/modules/ximgproc/samples/seeds.cpp:32:6: warning: no previous prototype for function 'trackbarChanged' [-Wmissing-prototypes]
/builds/precommit-contrib_macosx/opencv_contrib/modules/ximgproc/samples/seeds.cpp:32:26: warning: unused parameter 'pos' [-Wunused-parameter]
/builds/precommit-contrib_macosx/opencv_contrib/modules/ximgproc/samples/seeds.cpp:32:37: warning: unused parameter 'data' [-Wunused-parameter]
Merge pull request #90 from bkueng/superpixel_SEEDS

Integration of SEEDS superpixel algorithm to opencv_contrib