Invalid displayhook for other kernels

[takluyver]

KernelApp sets sys.displayhook to ZMQDisplayhook, which sends execute_result messages without an execution_count, crashing jupyter_console.

We don't see this in our own code because ZMQInteractiveShell then overrides it with ZMQShellDisplayHook, which works correctly. But this is an issue e.g. when running the xonsh kernel.

[scopatz]

Thanks for hunting this down!

[takluyver]

#162 should fix it.