Improve/request handler

Roll back to export a WPCOM instance because if not we can't extend it.

This refact doesn't change the API but add a way to pass the request handler in class constructor.

```js
var WPCOM = require('wpcom');

// Set CORS request handler passing token
var wpcom = WPCOM('<token>');

// Pass request handler
var wpcom = WPCOM(require('request-hanbler'));
```