ENH: In notebook, default repr of Frame is PNG.

I think the default display of a `Frame` in the notebook should be an image.

This does not change the output `print`, `repr`, `IPython.display_pretty`, and it does not affect any computations on a Frame. It only means that, in a notebook, the expression

    frames[0]

displays a nicely sized image.
It looks like `six` does have an alias for this, but `io.BytesIO exists in both 2.6 and 2.7, and I am not interested in supporting earlier version of python so this is fine.