[example] finetuning CaffeNet on Flickr Style data

This example shows how to fine-tune the BVLC-distributed CaffeNet model on a different dataset: [Flickr Style](http://sergeykarayev.com/files/1311.3715v3.pdf), which has style category labels.
The dataset is distributed as a list of URLs with corresponding labels.
Using a script, we will download a small subset of the data, split it into train and val portinos, and use the ImageDataLayer to load images directly from the filesystem.
Include a comment like "set to zero to disable learning for this layer" to echo the text.