Adjust export for Common.js Node-like environments.

Currently, when you require **DOMPurify** in Node.js environments or similar (like Browserify) an object containing `DOMPurify` is being exported and not the `DOMPurify` object itself. This small change allows this cleaner require: 
```javascript
var DOMPurify = require('dompurify');
```
Instead of the current:
```javascript
var DOMPurify = require('dompurify').DOMPurify;
```
Here's a working example on RequireBin: http://requirebin.com/?gist=a597860855d59cadfaeb