Auth Changes For Statsd Backend

* This module now gets a token at the first flush from Cloud Identity if Rax Username and Api key are supplied.
It uses that token for all posts to the CM endpoint. 
* If it gets a 401 from the CM endpoint, it again queries Identity for a new token. It then replaces the x-auth-token header with the new token.
Also, it's not explicit what the args to `callback` are. Idiomatically, a callback typically expects a [possibly null] `Error` object as its first argument. That means that on success you end up calling `callback(null, token)` and on error `callback(err, null)`.

Node.js for you. I don't make up the rules, I just play the game.