Add block break event (in Chunk) for mods, to fix #25.
Move break block event to World from Chunk

Fixes the event block coordinates, although now it has to be
hooked in two places.
Attempt at excluding unwanted break events

Known issues:

1. Players breaking mod blocks generates unexpected events
Since the stack trace includes the mod name, players breaking
modded blocks are recognized as the mod breaking the block.
Mods can and do override removeBlockByPlayer from Forge, so
they will appear in this stack trace. Tested with RedPower lamps.

2. Buildcraft quarries generate infinite items
The quarry tries to mine the blocks, and is able to drop the
item stacks (not as item entities, but to insert into the pipe
output), even if the set block is denied. See: https://github.com/BuildCraft/BuildCraft/blob/master/common/buildcraft/factory/TileQuarry.java#L410
worldObj.setBlockWithNotify() is called after mineStack(), and
its return value is ignored, so quarries repeatedly output the
item stack, even if we deny the block break.

3. ComputerCraft mining turtles leave trails of turtles
The mining turtle breaks block through TurtleTool, which can be denied..
however, when moving it replaces itself with air, in TileEntityTurtle.
Denying this block set causes the old turtle to persist, resulting in a
trail in the turtle's wake.

Still a ways to go..