Classifiers Updates

[cvalenzuela]

Renaming the ImageNet class to ImageClassifier following #98

This PR also add supports for loading and saving models trained with the KNNImageClassifier: #45. So now this is possible:

const knn = new ml5.KNNImageClassifier();

// Will prompt the user to save a json file
// Accepts one arg (the name of the file or defaults to Date.now())
knn.save(); 

// Loads a json file that was saved with .save() 
knn.load('sometrainedmodel.json'); 

The examples repo has another update following this too.