Added support for spawning small fir trees in snow biomes.  Made jungletrees
and palms require at least +15°C to grow.  This is supposed to keep them away
from snow biomes, but the engine doesn't let me tell the snow mod to always
run first.  Also please note that due to the load that snow biomes + moretrees
adds to the map generator, well... stuff will look cut off, broken, etc. due
to bugs in the map generator.