Documented API calls' return format.

This is in response to issue #108.

:ghost: :jack_o_lantern: :ghost: 
I think it's worth being specific about what the object contains here -- specification by example leaves something to be desired. Something like:

> The object will have at least the following fields:
> * `"name"`, which is the name/label of the node (string).
> * `"free"`, which indicates whether the node is free or has been allocated to a project.
> * `"nics"`, A list of nics, each represented by a json object having at least the fields:
>   * `"label"`, which is the nic's label.
>   * `"macaddr"`, which is the nic's mac address.

I put the words "at least" in there so that we can add to this later without breaking existing clients. @gsilvis, @lokI8, does that seem sensible to you?

Note: we should be more consistent with our terminology throughout: let's either do name or label and extinguish the other word from the codebase. (Just reported this as #312)