Add thumbnails for Image and File collections.

Image collections get first collection member's thumb and file collections (as well as file objects) get fake cover.

Closes #716 

#### Collection objects
![file-image-colls](https://cloud.githubusercontent.com/assets/96776/4158433/95cbdbbc-348c-11e4-8884-704860651edc.png)

#### File objects
![file-objects](https://cloud.githubusercontent.com/assets/96776/4158431/92b9849c-348c-11e4-8ceb-c17e861777f4.png)