New cobble / mossycobble/ ladder textures

The current cobble texture is completely wrong, for two logical reasons : 
- Its stone is way much darker than the raw stone and the stone bricks ones. Inappropriately darker. When you break the stone, its parts become suddently darker ? It's not realistic at all.
- Its lines that delimitate the parts are vulgarly rectangular. When you break the stone, the parts are always squarred like that ? Again, it's not realistic at all.

IMO, it's urgent to replace this poor draw.

So, I re-worked the cobble texture. Such as the stone bricks and the raw stone, it keeps the same range of colors / pattern (with some tweaks of course) - with a better fracture. In this pull, I also updated the ladder texture.

Before / after : 

![cobble](https://cloud.githubusercontent.com/assets/9265590/4660876/cd97e440-551c-11e4-878e-6f0d84b27d18.jpg)