* first try on OpenCV face

* find a way to visualize detected face; enlarge by 1.5x

* revision for first-time pull request

* faceDetector

* fix little mistake

* Update headers.hpp

* Update CMakeLists.txt

* OpenCV face detector thread save & 4x faster

* Added doc

* Exposing first hand heatmaps through handExtractor

* Hand extractor exposes heatmaps for both hands of all detected persons.

* Clean-up hand heatmap code

* Face heatmap extraction similar to the hand-heatmap code

* Update faceExtractor.hpp

* Update faceExtractor.hpp

* Update handExtractor.hpp

* Update faceExtractor.cpp

* Update handExtractor.cpp

* Face heatmaps added to wrapper

* Added hand heatmaps to wrapper

* Cleaned code

* Added description in doc