External shipping

Adding support for external shipping providers
Btw needs some copyright notices - just copy paste it from one of the other classes. All open source classes that have logic in them kinda have to have these.

Something like this:

```
/**
 * @package    Openbuildings\Purchases
 * @author     {name} <{github email}>
 * @copyright  (c) 2014 Clippings Ltd.
 * @license    http://spdx.org/licenses/BSD-3-Clause
 */
```