Added Silver tools

Added:
Silver:
Sword, Shovel, Pickaxe, Axe, Hoe

Changed the names for the textures.  (from "oreSilver" to "silver_ore"

Changed some textures
Silver ore is now less rare in world generation.

Silver tools properties:
Mining level: up to Iron ore.
Uses: Less then Iron tools, more then Stone tools
Speed: A bit faster then Diamond tools
Damage: 1 dmg more then Diamond tools
Enchantment luck: Higher then Diamond tools, Less then Gold tools