IOPub Messages Parent

[guysv]

Right now, IOPub messages have no parent, because such implementation is trivial.
This works for Jupyter-Console, but not so much for Jupyter-Notebook, and probebly other frontends, which seem to drop IOPub messages without parent headers.

One trivial solution: Save the parent header as a property in the kernel object, and let the user send it via kernel.send_update(..., parent)
I tested this; it allows txkernel messages to be displayed in the notebook, but this is a very hackish solution.
Theoretically, the kernel object is asynchronous, therefore it could handle other messages before finishing the previous message. Such scenario will break the 'self.current_parent' paradigm.

Another solution: create an external handler-object, which will be created for each request from the frontend. there the parent header will persist as a property, and multiple requests could be handled

[guysv]

c299675 implements solution one.