KNN Classifier (#171)

* added KNNClassifier class, and getFeatures method in ImageClassifier

* added knnclassifer.loaddataset, and savedataset from/as json file

* added featureExtractor.infer() to take in p5 videoelement

* support callback, and add 'label' for each class

* added getClassExampleCountByLabel, support label when save/load dataset

* update function naming, predictClass to classify, classLabel to label

Author: yining1023