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
precomp.hpp should contain at least opencv2/xobjdetect.hpp, but can also contain commonly used across the module headers, such as vector, string and such.