Floating Island World Generation

Minor update:

• Added config options to change the chances of each island type
  
• The spawn will be set to be on an island with trees if the current spawn is invalid
  
• Added a command (/islegen) to check on the plugin and the various config settings, available to all users, and with some sub-commands
  
• The plugin now checks for updates when reloading and every 15 minutes after that (by default), notifying admins if any are found, and shortening the link with bit.ly so you can click it in the MC chat without losing half the link