Added _order field to the stacked inline template

This change enables dragging of stacked inlines for models that subclass mezzanine.core.models.Orderable.
![screen shot 2014-05-08 at 12 58 33 pm](https://cloud.githubusercontent.com/assets/828125/2918613/07bf0974-d6d2-11e3-9bc8-7e41cd5fc4c1.png)