Add dummy /testfor command in Bukkit. Addresses BUKKIT-3813

This command only functions in command blocks so the bukkit command for it
simply spits out an error message.
Pull Contributing Guidelines and Requirements into CONTRIBUTING.md
Add link to CONTRIBUTING.md in README
Add missing new line to README.md
Remove point about squashing commits.
Add an enum for Nether Wart growth stages. Adds BUKKIT-1599
BlockState for Command Blocks. Adds BUKKIT-3805.
Pulling all pending Bukkit-JavaDoc changes
Update Bukkit for Minecraft 1.5.1
Added the hasGravity method to Blocks. Adds BUKKIT-3832
Add Fish (Hook) entity to PlayerFishEvent. Adds BUKKIT-1025
Add method to get the source of a TNTPrimed.  Adds BUKKIT-3815
Add ability to modify ThrownPotion properties. Adds BUKKIT-3197
BlockBurnEvent should know what caused the ignition