ftw.activity integration

Add more content to activity stream of ftw.activity for ftw.file content:

- Display small preview when it is an image
- Display download link when it is not an image

![bildschirmfoto 2014-09-03 um 14 03 53](https://cloud.githubusercontent.com/assets/7469/4133734/87c0e2f4-3362-11e4-968f-abc026a8278d.png)

@maethu