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/bucket/README.txt

@@ -1,4 +1,4 @@
-Minetest 0.4 mod: bucket
+Minetest Game mod: bucket
 =========================
 
 License of source code: