Fix IPython.utils.ansispan() to ignore stray [0m

Previously, `IPython.utils.fixConsole("\033[0m")` (I use `fixConsole()` because `ansispan()` is private) gave `"</span>"`. I also changed `ansiyellow` to something that looks actually yellow (but still dark). I'm submitting this to `master`, but it would be good to backport this to 2.x (whatever the procedure is).