removed all code that tries to disable default trees because the minetest dev
team is being completely obstinant about letting me properly disable them from
this mod.

Instead, made beech trees respond to enable flags in settings.lua (disabled by
default).  If you want beeches, turn that flag on in settings.lua and remove
the "trees" flag from yourworld/map_meta.txt in the mg_flags line.
Made settings world-specific - look for moretrees_settings.txt in your world
folder.  If it isn't found, the mod will create it with the contents of
default_settings.txt and then read those default settings and use them.
Otherwise, it will read its settings from the world folder.
forgot to test if the file can be read the second time the mod is run.
Should be fixed now.
Merge branch 'master' into newstuff