Adding chainability to p5 DOM Element methods.

As explained in https://github.com/lmccart/p5.js/issues/227#issuecomment-51721245

Addind ability makes **p5js** more *jQueryesque*. It should make learner to think more in mode `<javascript>` than in mode `<processing>`.

Lastly, I didn't find DOM library in the `src` folder so I worked directly in `lib/addons/p5.dom.js`. Make me know if should have done otherwise.