Feature jpeg loader thanks to WebARKitLib

[kalwalt]

This PR add these features:

• [x] WebARKitLib as a git submodule.
• [x] jpeg loader.
• [x] package.json file and script makem.js to build WebARKitLib with emscripten.
• [x] improved shell script ( build.sh )
• [x] debug version of libar.
• [x] improved examples where using a jpeg image.

[kalwalt]

I will not update the grayscale_rgba_example.html because the loadJpeg function load RGB data not RGBA and the final image will be not well formed. In the grayscale_rgba_internal: https://github.com/kalwalt/webarkit-jsfeat-cpp/blob/1b9082355f4c778a9c7f88884d16d6e090f18f78/src/imgproc/imgproc.h#L178-L197 it is requested to add a different count for RGB: https://github.com/kalwalt/webarkit-jsfeat-cpp/blob/1b9082355f4c778a9c7f88884d16d6e090f18f78/src/imgproc/imgproc.h#L195 for RGB it should be 3. We can add a new argument in the function to check the color space and so the routine can use a different counter.