Add support for MarketplaceWebServiceOrders API

```
Using basically the same implementation does seem possible to abstract both 
libraries to a basic Client and Model implementation and from that apply 
changes to other APIs than v20090901.

Orders library was added to trunk as a reminder.
```

Original issue reported on code.google.com by `m...@phpkuh.de` on 25 Aug 2011 at 8:45
