Updates for OpenCV 3 api (keeping OpenCV 2 back compatibility)

Updates for OpenCV 3 api (keeping OpenCV 2 back compatibility).

Tested with:

• OpenCV 2.4.9
• OpenCV 3.1

(working with both 2 and 3 major versions).

Main changes:

• New paths for includes.
• Detectors/descriptors new api.

src/examples/test_ROI_ORB.cpp

 //std includes
 #include <iostream>
 
-//opencv includes
-#include "opencv2/features2d/features2d.hpp"
+// OpenCV includes
+#if defined (HAVE_OPENCV3)
+#include <opencv2/features2d.hpp>
+#include <opencv2/highgui.hpp>
+#include <opencv2/core.hpp>
+#include <opencv2/imgproc.hpp>
+#else
+#include <opencv2/features2d/features2d.hpp>
 #include <opencv2/highgui/highgui.hpp>
-#include <opencv2/imgproc/imgproc.hpp>
 #include <opencv2/core/core.hpp>
+#endif
 
 int main(int argc, char** argv)
 {