Beds : king size extension

CAUTION : code incomplete, unoptimized and buggy.

Overview of the pull-request : 
- Models & textures : OK ✓
- Bed "king-size" extension : works only on one facedir, and bug when foot/head are next (on X).
- Bed "king-size" unextend : completely ineffective.
- Single bed placing : buggy on 3 facedirs.

![bed_king](https://cloud.githubusercontent.com/assets/9265590/4871889/5a9838fc-61cc-11e4-98d8-f4d9a8a43d26.jpg)