Initial cGridStructGen refactoring.

Ref.: #987.
Initial cPrefabPiecePool refactoring.

Ref.: #986.
cNetherFortGen uses cGridStructGen.

WIP, this doesn't work properly yet.
Fixed MSVC 64-bit build warnings.
Merge branch 'master' into GridStructGen
Fixed cGridStructGen.

Now cNetherFortGen works with the new architecture.
Mineshafts generator rewritten to use GridStructGen.
cWormNestCaves rewritten using cGridStructGen.

Ref.: #987.
Ravines rewritten using cGridStructGen.

Ref.: #987.
Removed an unused macro.
Removed an unused NetherFortGen variable.
Removed unused constants from MineShafts and Ravines.
Merged branch 'master' into GridStructGen.
Each time a portal block receives a tick it has a 1 in 500 chance of spawning a zombie pigman.
A portal block now chooses a number between 2000 and 0
Fixed compile problem.
Merge pull request #995 from mc-server/Portal_Pigman

Portal zombie pigman
Fixed the console saying no plugins are loaded.
Merge pull request #993 from mc-server/GridStructGen

Grid struct gen