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
looks like the architecture is not very good. "train" does not take feature evaluator, "predict" does take it. train returns some vector, which meaning is not very clear. Maybe, we should have "bool train(...)" and then "void getFeatureIndices(vector<int>& vidx) const"; ?