how to compute priorBoxes in  HouseNumberDetection example

#### Issue Description


- As I know, before training yolo2, we need provide  priorBoxes for our model, but the HouseNumberDetection example  did not explain how to compute it. I  hope dl4j can provide some method that can compute priorBoxes by k-means algorithm base training data.


#### Version Information



* 1.0.0-beta2



