Patch abernier bunchofthings

[abernier]

Among:

• global srcset object
• update all images at once: srcset.update()
• update one image at a time: srcset.imgs.get(<img>).update()
• a 'srcchanged' event dispatched on the <img> when changing src
• data-srcset favored in order to keep backward compatible with the new spec (especially in chrome which was complaining about Failed parsing 'srcset' attribute value since it has multiple 'x' descriptors or a mix of 'x' and 'w'/'h' descriptors.)
• new simple build system based on browserify