Support the matplotlib widget backend

[blink1073]

Thanks to the ipykernel@4.7 release, we can now use:

%matplotlib widget
import matplotlib.pyplot as plt

plt.plot([0, 1, 2, 2])
plt.show()

[stonebig]

remark: if you try to update the top cell in this example, this is the bottom cell that is re-drawn.

[tacaswell]

@stonebig That may be the correct behavior depending on what the 'current' axes is.

[SylvainCorlay]

Closing as rebased in #54.