Add EntityLeash and ItemLeash for diff visibility
Implement Leash API. Adds BUKKIT-4459, BUKKIT-4583
Update players when coming into view of vehicle. Fixes BUKKIT-4628

When only considering trackers from player perspective, attach entity
packet could be sent before a packet for a respective vehicle is in view
and will, in turn, be ignored.

This adds another notification when the vehicle comes into view to cover
all cases.
Fix mistranslation in isPlayerCreated() for IronGolems. Fixes BUKKIT-4543
Fix off by 1 error for spawn protection. Fixes BUKKIT-4154
Updated version to 1.6.2-R0.1 in pom.xml for Beta.
Updated version to 1.6.2-R0.2-SNAPSHOT for development towards next release.
Merging from upstream