Add spec for setting faxes attribute to empty array
Add slug to Category

This will allow categories to be updated from the admin interface via their slugs as opposed to their DB IDs, which are not guaranteed to always be the same, especially when testing both the API and admin interface locally. The specs for the admin interface currently communicate with an instance of the API running locally on the same machine as the admin interface.
Add "/children" endpoint for a category

This makes it much easier for a client to get any category's children by exposing the results of the "children" method of the mongoid nested set.
Use Category Entity when returning all categories

The default attributes returned contain many unnecessary ones. With Grape Entity, we can specify only the ones we want to return.