skip_known_failure using skip, not knowfailureif
skip_known_failure using skip, not knowfailureif

Thomas and I looked into getting the known failures to show
up as 'K', instead of 'E', but it involved grabbing a custom
nose plugin from numpy and didn't seem worth the trouble. So
I'm putting skip_known_failure back to use skip and show up
as 'S'.

ipython/ipython,ipython/ipython