Added rarity filter for Crafting Bases

Fourth parameter in crafting_base.txt is rarity (White, Magic, Rare,
Unique).  Rarity filter is == rather than >= like the other filters.

Multiple entries of the same item name are now handled correctly using a
List instead of Dictionary for crafting bases.  This allows for ex: Gold
Ring,75,0,White and Gold Ring,75,0,Rare to both be in the file and
prevent Magic Gold Rings from showing up.