#32 add support for attributes

I Don't know how to document this.

The way it works :

```
api = trading(appid=settings.EBAY_APP_ID,
              devid=settings.EBAY_DEV_ID,
              certid=settings.EBAY_CERT_ID,
              domain=settings.EBAY_DOMAIN,
              token=settings.EBAY_AUTH_TOKEN,
              siteid=siteid)

api.execute('YourAction', {'some': 'datas', 'like': 'before', 'but': {'@attrs': {'you': 'can', 'add': 'attributes'}, '#text': 'and text' } })

api.call_xml
> '<but add="attributes" you="can">and text</but><like>before</like><some>datas</some>'
```

You can pass one or many attributes (you should pass a dict when your key is @attrs), and if you need to have text in a node with attributes, you can use #text. 

Seems to be good enough. I'm just not sure about how document it.