Add poplar tree

Poplars exist in two kinds: small and large trees. Both use the same
nodes, except for the sapling. Poplar leaves may drop one or the
other sapling.

Poplar trees require a humid environment, and water nearby.
Provided there is water, an occasional poplar will grow under less humid
conditions.

default_settings.txt

@@ -15,6 +15,7 @@ moretrees.enable_birch			= true
 moretrees.enable_spruce			= true
 moretrees.enable_jungle_tree	= true
 moretrees.enable_fir			= true
+moretrees.enable_poplar			= true
 moretrees.enable_beech			= false
 
 -- set this to true to make moretrees spawn saplings at mapgen time instead