Vcard based avatars


![serve_avatar_presence001](https://cloud.githubusercontent.com/assets/652428/5222493/57639550-7699-11e4-9724-25c34d195a0d.png)
![serve_avatar_presence002](https://cloud.githubusercontent.com/assets/652428/5222494/57660dbc-7699-11e4-89cc-31977c3873c2.png)

**TODO**:

- [ ] missing unit tests

serve diaspora avatar via presence update related to #43 
ah grrrr ... it is always the same. you post a problem and instantly find the solution:

this one is working:
```
node.root.xpath("//xlmns:x", 'xlmns' => 'vcard-temp:x:update')
```