Adjust export for Common.js Node-like environments.

[djfarrelly]

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:

var DOMPurify = require('dompurify');

Instead of the current:

var DOMPurify = require('dompurify').DOMPurify;

Here's a working example on RequireBin: http://requirebin.com/?gist=a597860855d59cadfaeb

[cure53]

Thanks!