Major rework of command handling. This is mostly internal code changes rather than something noticeable to the end user, though on command input errors (wrong number or format of arguments, etc.) it will now have it show examples for the command the same as seen in the command list provided by the root wb command.

Replacing the monolithic and crude "if...else if...else if..." command handler in WBCommand.java with every command crammed in there, each command is now handled in a separate subclass in the new "com.wimbli.WorldBorder.cmd" package. This is inspired by the way the Factions plugin handles subclassed commands.

This is the first pass of the command handling rework. I still need to do some further testing, and I plan to further improve feedback for input errors to also show some basic help info along with the command examples.