Node compatability

* [ ] get rid of window export
* [ ] use lib for Promise object
* [ ] use lib for XMLHttpRequest object
* [ ] 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;
```