[B] Add an enum for Nether Wart growth stages. Adds BUKKIT-1599

Currently, there is an enum or other class to handle the growth state for all other plants - Crops, Cocoa, etc. There isn't one, however, for Nether Wart, which means that you have to use raw byte data to handle these instead of a formatted name.

Associated JIRA Ticket - https://bukkit.atlassian.net/browse/BUKKIT-1599