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.
Also improved the look of both a little bit, and re-balanced the spawn/growth
chances, and reduced the wait time sicne we're not using a ton of code to do
this anymore.
Also made the spawn routine clear out any leaves that are in the way of a
spawning tree's trunk, so the trunks don't look like they've been cut by
leaves.
