Added a _refName property when converting Firebase object to an array

Hi, 

I was playing with this awesome ReactFire library, and I ran into this issue:

I used the `bindAsArray` method to bind a list of items to a Firebase ref. And then when rendering that list I needed to have a `key` property for each item in it. I saw some people using the index variable of a loop to create this `key` property, but according to React's documentation (http://facebook.github.io/react/docs/multiple-components.html#dynamic-children) I think that's not the proper way of doing it.

So I thought it'd be nice to get the unique ref name of each item in the bound array (maybe something named `_refName` to avoid potential conflicts) to be able to use a simple `key={item._refName}`.

Cheers!