Floating Island World Generation

Minor changes:

• Added tall grass and flowers to the list of erasable blocks (things like dungeons hated them)
  
• Rounded island spawn chances to 2 decimals
  
• Update checker will no longer do an on load update check if the delay is set to 0 or less (got told off for having it forced :P)

Gravel patches can spawn on the bottom of sand islands allowing the sand to fall, will make special code for gravel patches.
Grass/flowers/snow can spawn mid-air when a cave/fluidpool removes the block under them but not the actual block. This will be fixed next version.