Added slab .OBJ files

in resources/materials/block/nonsolid/slab/..

3 kinds :
-bot slab
-top slab
-top sandstone slab (because the texture is different)

Files are made to be used with only one texture file for each block.
Just have to change the texture in the smt.

Signed-off-by: Thomas LAMSON <tomlabete@numericable.fr>