support for firebase's push()

[kylemac]

I have an untested branch that has this working, but wanted to know if you'd be interested in this before we pose a PR.

in short, firebase has support for saving lists of data via their push()function (as opposed to update()). this allows us to tack on object for a user has many <foo> date. right now the save()function only accommodates a regular firebase update()

since you need to get a reference to the original child object first, the proposed interface is:

// wherein `entry` is a new has-many object 
// and `userId` is the child it will be pushed to
controller.storage.users.push(userId, entry)