Use entity UUID instead of EntityID for metadata. Fixes BUKKIT-3582

The entity id is a non-unique, non-persistent value, and will cause
entities to lose their respective meta data on chunk unloading, and
teleportation.
Set CraftTravelAgent.DEFAULT to a non-null value. Fixes BUKKIT-3578

Currently, CraftTravelAgent will call s() on the passed-in WorldServer in order to set DEFAULT. However, s() will always return null at this point, because WorldServer.P will still be null, as it is set after the constructor is called. Instead, we set CraftTravelAgent.DEFAULT to the instance that is being constructed.
Check TravelAgent.findOrCreate(Location) for null; Fixes BUKKIT-3590

If a custom TravelAgent is used and returns null for findOrCreate method
a NullPointerException will occur.
Conflicts:
	src/main/java/net/minecraft/server/PlayerList.java
Close inventory on teleport. FIXES BUKKIT-3021.
Added the unit test for hasPhysics in org.bukkit.material.
[B+C] Implement a test for the method hasGravity for Blocks. Adds BUKKIT-3832

Create a unit test for the PR implementing the feature :
B-769 - https://github.com/Bukkit/Bukkit/pull/769 - hasGravity method
Merge pull request #2 from nitnelave/Material

Material