Online state fixs #82

As discussed in #82. 
I have made the avatars for group conversations 40px, so the UI is more consistent.
![signle](https://cloud.githubusercontent.com/assets/2996275/3887017/65354674-21e9-11e4-82a8-f8fbe56f0733.png)
![group](https://cloud.githubusercontent.com/assets/2996275/3887018/65e9c388-21e9-11e4-8d5e-2eb5e6e64293.png)

Please review @jancborchardt @fossxplorer @stefan-niedermann