modify README.md
Change wget download option from -o to -O
fix bing_wallpaper.sh
fix picName which is extracted from picURL, making picture be saved
exactly in saveDir.
modify the path invoking kde4_set_wallpaper.sh in order to invoke it
correctly wherever you execute bing_wallpaper.sh. Sure,
bing_wallpaper.sh and kde4_set_wallpaper.sh must be in the same
directory.