Adds BUKKIT-4780, BUKKIT-4781. Makes Pumpkins, Melons, Carrots and
Potatoes extend Crops.

With the recent deprecation of getData and setData, there is no forwards
compatible way of accessing the growth state of Pumpkins, Melons,
Carrots and Potatoes. As they are all farmable plants with 8 stages of
growth, it seems reasonable to make them extend Crops, which provides
accessors and mutators for their growth using CropState instead of
depracated magic values.