parseFromString now renders entirely plain text documents as textNode

This pull request resolves the issue where parseFromString throws error
when given plain text to parse.

Summary of issue:

This example code throws the following error: `source code out of
document root`

```js
var xmldom = require('xmldom'),
    DOMParser = xmldom.DOMParser,
    XMLSerializer = new xmldom.XMLSerializer(),
    string = 'plain text',
    doc = new DOMParser().parseFromString(string, 'text/html'),
    s = XMLSerializer,
    str = s.serializeToString(doc);
console.log(str);
```

The resulting document given is this:

```js
{ doctype: null,
  implementation: { _features: {} },
  childNodes: {},
  documentURI: undefined }
```

It should return a single textNode childNode instead of an empty set of
childNodes.