Use net.minecraft.entity.Entity.teleportTo() in CraftEntity.teleport()

Instead of manually clobbering the position and world, actually use the mostly-stable teleportTo function already in the base code. This has the benefit of working for all tested cases so far (minecarts and thrown items).

You can use my plugin Nethrar to play around with it yourself. There is a special MCPC+ branch which we are currently using for our server, running the 1.6.4 release of MCPC+. You can clone it from https://github.com/akrieger/Nethrar/tree/mcpc+ and trivially build it using 'make', after editing the API variable in the supplied Makefile.generic to point to the location of the MCPC+ API jar.