limited compatibility with IPython 3.2

[smason]

I've been struggling with iPython 3.2 compatibility and have tracked it down to the changed difference: execute isn't an attribute of the shell_channel.

Seems that since the Jupyter/IPython division ipykernel should be used instead, but this change gets it sort of working.

[kenny-evitt]

@smason Thanks for the PR.

@hayd @UnknownProgrammer Would you mind reviewing this change?

[smason]

Huh, iPython 4 has just been released! May be worth doing the proper things now?

[kenny-evitt]

@smason If you can help me test your changes, or I can find others to do so, I'll gladly merge PRs to "do the proper things". Let me know how I can help.