Improve Device Report

![img](http://tmp.dbrgn.ch/screenshots/20140506_213200.png)

### Default values

 We should unify the "default values" for non-existing or empty fields or foreign keys:

![img](http://tmp.dbrgn.ch/screenshots/20140506_213256.png)

I'd probably vote for `None` or `-`.

### Sessions Paging

The sessions should be paged. With paging, the 50 items limit is probably not needed.