Change image link (now relative)

https://github.com/blog/1395-relative-links-in-markup-files
A horizontalOffsetPlane for things like train tracks and whatever.
Merge remote-tracking branch 'movingblocks/develop' into smallBugfixes
Terasology. New block family. Fences, rails and lader as bonus
Merge #438 from @AWFeldick first even though it also exists in #446 :D
Merge #446 from @small-jeeper for auto-adjusting fences, rails, and a ladder!
Merge #444 from @mkalb to make an image link relative
Tiny bit of cleanup/commenting