Freeform cart v4

Reformatting changes.
One pattern I've used to simplify this would be to create a _setAttributes method that contains all of the calculated attributes that need to be set on the model. This way, calculated attributes don't have to become function calls.

Something like this:

initialize: function() {
  this._setAttributes();
},

//named with underscore to indicate a private method
_setAttributes: function(){
  this.set('subtotal', (this.attributes.price * this.attributes.quantity).toFixed(2), , {silent: true});
  //Add more calculated attributes here.
}