Bed model : wider & simpler

- Wider mattress : 75 cm (unrealistic, too narrow) -> 87.5 cm.
- Removed the nodeboxes planks under the bed : wasn't really necessary and it still looks good.

![screenshot_147640949](https://cloud.githubusercontent.com/assets/7883281/4611587/bba4a88e-52bc-11e4-80ab-2a64c22ed5ac.png)
![screenshot_147601071](https://cloud.githubusercontent.com/assets/7883281/4611586/b901443e-52bc-11e4-9ed7-263de2704c8a.png)