Undefined 'order' model attribute

"Todo model" section in "Exercise 1" chapter mentions that the sample Todo Model has an `order` attribute, while in fact it doesn't.

I guess this is legacy from an early version of the sample, as from [practical's script.js](https://github.com/dprotti/backbone-fundamentals/blob/2e43d7460750c2b6a02d93f52420cf2aea385ea3/practicals/stacks/option1/public/js/script.js).