Testing new WebARKitLib Trackers

@ThorstenBux @EdwardLu2018 i'm testing a new class that i created inside WebARKitLib see here in my testing branch feature-webarkittrackers . You can see the WebARKitOrbTracker.cpp that implement the necessary functions for initializing and to track.
WebARKitOrbTracker is a class, but i think we should implement a base class that may serve as base class for all future trackers. I also created a WebARKitUtils.h file to store functions that can be shared by trackers. For example we could store the im_gray(uchar data[], size_t cols, size_t rows) and maybe other functions.
There is also an example, but it only provide a test to verify the setup. It is just a beginning, a lot of work must be done! 😄
At least i can say the project may be built without errors, from emscripten....