Add/desk - adding integration for Desk.com

Adding index.js and associated tests for Desk.com
I think it'd be cooler if we calculate:

```js
var url = this.siteUrl.replace(/<siteName>/, settings.siteName);
```

in each one of the request methods instead (`_getUser`, `_createUser`, `_updateUser`).  That way we can reduce the signature of each of those methods by removing `url`.