Fixes #34: createTextNode now coerces argument into a string

Two commits: a test and a fix.  Let me know if anything needs to be tweaked.

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For what it's worth, I really like domino for doing HTML document manipulation in Node with jQuery.  And it seems to work with very little effort.

```
var jquerySourceCode = fs.readFileSync('jquery-2.0.3.js', 'utf-8');
var window = require('domino').createWindow('<html><!-- some html --></html>');
(new Function(['window'], jquerySourceCode))(window);
var $ = window.$;
// Use jQuery!
```

Many people complain about the pain of setting up jsdom for use with jQuery or Zepto, but for everything I do domino works really well.