Rename to Minetest Game

Rename in game.conf and documentation
Update game_api.txt documentation for bucket API and tree functions
Fix tab, space and comment formatting in game_api.txt
Rename in mod READMEs

mods/boats/README.txt

@@ -1,6 +1,6 @@
-Minetest 0.4 mod: boats
-=======================
-by PilzAdam, slightly modified for NeXt
+Minetest Game mod: boats
+========================
+by PilzAdam
 
 License of source code:
 -----------------------