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 
the whole looping is ugly... in theory I could do it with xpath on a single line but I have some problems with getting it working.

checkout my test setup:
```
2.1.2 :001 > require 'nokogiri'
 => true 
2.1.2 :002 > node = Nokogiri::XML('<presence from="test2@lmatt09.intranet.astaro.de/0572a359-c43b-47d4-bfa1-259807203161">  <priority>1</priority>    <c xmlns="http://jabber.org/protocol/caps" node="http://pidgin.im/" hash="sha-1" ver="lV6i//bt2U8Rm0REcX8h4F3Nk3M=" ext="voice-v1 camera-v1 video-v1"    />      <x xmlns="vcard-temp:x:update">          <photo/>            </x>            </presence>')
```

now if I am executing a simple search on the priority:
```
2.1.2 :005 > node.root.xpath("//presence/priority")
 => [#<Nokogiri::XML::Element:0x175e198 name="priority" children=[#<Nokogiri::XML::Text:0x18c3f9c "1">]>]
```

it works well, but if  I am starting to search for `x` or `xmlns` it will not work:
```
2.1.2 :007 > node.root.xpath("//presence/x")
 => [] 
```
or
```
2.1.2 :010 > node.root.xpath("//*[@xmlns='vcard-temp:x:update']")
 => [] 
```

any ideas on that?