Rock Salt

Salt is a pretty common substance added by other mods. Usually, it is get by evaporating water, mining sand or as an ore. However, Halite isn't really a type of ore in the way iron or diamonds are. Thus, I though that adding Rock Salt as a type of Sedimentary rock to UBC would be great way to make UBC compatible with those mods that add salt, in a UBC way.

Note: This pull request only contains the textures for rock salt, as I'm not very experienced with adding blocks