ADAS: training app, stumps and waldboost

Training app, stumps and waldboost implementation. Stumps are tested, waldboost will be tested this week with feature computation.

For contrib application to compile, apply this patch
https://github.com/shahurik/opencv/commit/2c613cf7ad0d4d2e4d727494df0232d4094e166f
and use command
```
cmake -DOPENCV_EXTRA_MODULES_PATH=<opencv_contrib/modules> -DOPENCV_EXTRA_APPS_PATH=/<opencv_contrib/apps> <source_dir>
```

ACFFeatureEvaluator and WaldBoost won't be public classes, should they be derived from Algorithm?

@vpisarev @nikman-ru
I think there is no point in storing indices, since they are needed only once after training: ICFDetector uses indices to get his internal feature description and after all these descriptions will be saved. Indices are useless during prediction if we don't know real features.