Disable -twisted on non-win32 platforms

IPython/Shell.py

@@ -1167,8 +1167,15 b' def _select_shell(argv):'
     special_opts = user_opts & all_opts
 
     if 'twisted' in special_opts:
+        if sys.platform == 'win32':
         import twshell
         return twshell.IPShellTwisted
+        else:
+            error('-twisted currently only works on win32. Starting normal IPython.')
+            special_opts.remove('twisted')
+            return IPShell
+            
+
     if 'tk' in special_opts:
         USE_TK = True
         special_opts.remove('tk')