Working on adding a whole new set of trees to the mod

Most of the added code written by RealBadAngel
Also adds a couple of additional biome controls to jungle trees and conifers.
Also adds a text file describing the biome settings in a human-readable manner.

At present, this doesn't actually add anything new, it just refactors the
code to allow for more trees, and adds a bunch of biome definitions, tree
models, textures, etc. but no code to use them, yet.
Enabled several new types of trees, all spawned at mapgen time!

This code will slow the map generator down a little due to bugs therein, and is
generally still in the experimental stages, but it works and is actually quite
fast!
updated the date in init.lua
Added rarity control to biomes, all default to 50% for now.
changed default rarity in example at top of biome_defs.lua to 50.
Added sapling spawning, tuned rarity of some of the trees.
disable debugging info, set sapling growth values to normal
(interval = 500, chance = 10)
fixed birch and spruce always appearing above their planted saplings
Added tweak to make sure planted saplings only grow on the same surfaces as
they would normally spawn on.
typo
Made surface-based sapling growth depend on surface value
 in each tree's biome
set sapling interval/chance to their defaults, enabled beech
tree code which replaces the default trees.
fixed missing spruce tree cones
disabled beech trees
removed saplings, added leaves to avoidnodes list