Implement Entity.isOnGround(). Adds BUKKIT-3787
Make CraftMinecartHopper work as InventoryHolder. Adds BUKKIT-3796
Implement InventoryPickupItemEvent. Adds BUKKIT-3798
Implement new cause versions of BlockIgniteEvent. Addresses BUKKIT-3609, BUKKIT-3656, BUKKIT-3657
Implement per-player Weather API. Adds BUKKIT-812
Only set player weather if a plugin requested it.

If the server changes the weather it will set the per-player weather
variable and future changes will not apply. We should only set this
variable when a plugin is requesting per-player weather and not when
the server it doing it.
Merge remote-tracking branch 'upstream/master'