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;
```
(I'm thinking about using github CDN to serve our js minified files, like we're doing but 1. minifying them and 2. using our own domain so we don't tie our users to github).