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.
it would be more idiomatic to include this function directly as a parameter to `autheticate(...)`. E.g.:

```javascript

authenticate(this.tenantId, username, apiKey, function(token) {
  // do the auth stuff here.
});
````