Craft Convenience is a mod aimed at rebalancing some of the features of the game. It adds several new crafting recipes, and alters some of the game's original recipes as well. The mod is based off a suggestion here.
Some mob drops have been modified:
Wither skeletons have a 10% chance to drop a skull instead of 2.5%
Horses will always drop at least 1 leather, rather than 0-2