Tree Decaying

added treedecay function to kill a tree after a length of time if its trunk is chopped/dug.  effectively it only kills from the break up! 

also added the mapgen stuff to support this function.
Single-character names like these are bad bacause they're hard to understand.  Use something like:
`p0` --> `pos`
`p1` --> `pos_under`
`n0` --> `node`
`n1` --> `node_under`
`d`  --> `decay`