Changes from mwkaicz including:
- Ships don't sink through the void anymore
- New craft options:
	canHover - For Hovercraft, which follow the contours of the land
	canHoverOverWater - Same as above, but over water
	moveEntities - if false, the craft will not move players or entites on
	it (IE: for torpedoes or rockets)
	useGravity - if true, the craft will fall over time
	hoverLimit - the distance above the ground the craft will hover to
	harvestBlocks - list of materials through which the hovercraft will
move 
	
I could not automatically merge this pull request, so I had to do it
manually, which lost all version information. Please note these changes
should be credited to mwkaicz, not me.