Added a few more natural blocks to the "transparents+natural" blocks.
Fixed a bug (spawn point of default world always used to generate the circular wall).

Also, fixed javadoc + code style.
Added a way for developpers to change the shape of the border.

(Circular/squared, using the UHBorderManager.setCircular(boolean) method.)

Also, updated Javadoc.
Fixed: possible incorrect world selected in the border check (previously using the first one).
Fixed a bug related to the reduction warnings (badly calculated distance).

For squared borders, the calculated distance was the good one on the east & south sides of
the border, but it was the good one + the "diameter" of the future border on the other sides.

The distance was correct in angles.