Image moments

Fixed some errors in the calculation of the image moments and started writing more detailed tests in the DescriptorServiceTest class. 

The DescriptorServiceTest is still WIP but for the image moments (moments, central moments, normalized central moments and hu moments) tests we're written and the output was compared to that of OpenCV (see [Moments](http://docs.opencv.org/java/org/opencv/imgproc/Imgproc.html#moments(org.opencv.core.Mat)) and [HuMoments](http://docs.opencv.org/java/org/opencv/imgproc/Imgproc.html#HuMoments(org.opencv.imgproc.Moments,%20org.opencv.core.Mat)) ).