Added attachment

Added the much-needed attachment configuration option. 
Now you can attach something to a Tweenable instance (e.g.: a DOM element, an object, a string, etc.) and it will be passed to start/step/finish methods as a second parameter. 
This is very useful. A simple example would be to attach DOM elements (and thus, caching them) to Tweenable instances, so, in the "step" method, you can manipulate/update the attached element's properties according to current state. (of course, more efficient update-batching mechanics would benefit improved performance, but that's another story, Shifty is just a tweening engine, it does not "know" of the DOM and the environment - and it's great!).
Demo available in "attachment-demo.html".

I think we can drop the `|| null`.  If `config.attachment` is falsy (0, empty string, false, etc.), then `this._attachment` will unexpectedly be `null`.  If `config.attachment` is not provided, `this._attachment` will default to `undefined`, which is just as useful as `null` for this scenario.