Make HopperMinecart implement InventoryHolder. Adds BUKKIT-3796
Add InventoryPickupItemEvent. Adds BUKKIT-3798
Add block or entity causes to BlockIgniteEvent. Addresses BUKKIT-3609, BUKKIT-3656, BUKKIT-3657
Add per-player Weather API. Adds BUKKIT-812
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