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
all such classes should derive from Algorithm, they should not have constructor, no data fields, no protected or private methods. There should be static method create() (or external function) that returns Ptr&lt;ACFFeatureEvaluator&gt; Please, see video module, background subtraction classes as example