Improve generic code quality

[lmammino]

Some hints received by the NodeJs community:

• Reduce the size of the Flickr class (http://blog.izs.me/post/48281998870/unix-philosophy-and-node-js)
• Flickr class methods getSetInfo and getSetInfoPage can be unified into a single method where getSetInfo is the base case that downloads the first page
• Use the EventEmitter wisely: emits new tasks when a page is loaded and start to download the images as soon as possible (avoid to block the entire flow while pages gets collected). Also see Redesign to leverage streams and better command line composition #22 .
• Use prototipe to attach functions to the Flickr class