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/creative/README.txt

@@ -1,5 +1,5 @@
-Minetest 0.4 mod: creative
-==========================
+Minetest Game mod: creative
+===========================
 
 Implements creative mode.