Fix error in %gui. Closes gh-160.

IPython/core/magic.py

@@ -3244,7 +3244,7 b' Defaulting color scheme to \'NoColor\'"""'
         we have already handled that.
         """
         from IPython.lib.inputhook import enable_gui
-        opts, arg = self.parse_options(parameter_s='')
+        opts, arg = self.parse_options(parameter_s, '')
         if arg=='': arg = None
         return enable_gui(arg)