New textures for main menu

-different logo(-text)
-menusplash.png added (dirt-with-grass and lightsport in middle)
-tested with build PilzAdams last build57cbb8b-oregen (https://dl.dropbox.com/s/mfcz2wy63yxvm2c/minetest_costum_menu.png), the little gap at the right is fixed.