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.