Node compatability

* [x] get rid of window export
* [x] use lib for Promise object
* [x] use lib for XMLHttpRequest object ([xhr](https://github.com/Raynos/xhr) ?)
* [x] update README

```diff
 var utils = require('./ext/utils.js');
 var console = require("console");
+var Promise = require("bluebird");
+var XMLHttpRequest = require("xmlhttprequest").XMLHttpRequest;
 var hawk = require('hawk');
 var deriveHawkCredentials = require('./ext/hkdf.js').deriveHawkCredentials;
```