added al_path_to_root and al_open_map_from

So I'm writing my code in Netbeans and it builds my binary to gamedev/dist/Debug/GNU-Linux-x86/ , but all my important stuff (.tmx files) are in gamedev so I'd have to call: al_open_map("../../../data/maps", "level1.tmx")

Not only that, but calling "al_change_directory(al_path_cstr(cwd, ALLEGRO_NATIVE_PATH_SEP));" on what was line 461 would mess up the rest of my imports (like fonts) because it would take me to dist/Debug/GNU-Linux-x86

So to sum it up, I wrote these two methods to:
    1) al_open_map("../../../data/maps", "level1.tmx") -> al_open_map_from("gamedev", "data/maps", "level1.tmx")
    2) fix al_change_directory to previously held directory