bukkit: buffer writing of block changes to world

To ensure that committing an EditSession to the world doesn't block
the main server thread for too long (causing clients to lose connection
to the server as a result), block changes will be written to the world
in a buffered fashion only.
bukkit: separate class for plugin config
bukkit: flesh out event handler code a little more
plugin.yml: more info about commands and permissions
bukkit: simple /sculpt command help