DOC: Use square plots for Pie Plots

[TomAugspurger]

They look funny because the aspect ratio is off: http://pandas-docs.github.io/pandas-docs-travis/visualization.html#pie-plot

May also want to note that in the text also.

This would be great for a first pull request if anyone is interested. Just need to set the figsize to something like (8, 8) for each plot (or (16, 8) for the subplots example). Everything is in doc/source/visualization.rst

[onesandzeroes]

As well as setting the figsize, you might want to try forcing an equal aspect ratio, e.g.:

series = Series(3 * rand(4), index=['a', 'b', 'c', 'd'], name='series')
ax = series.plot(kind='pie')
ax.set_aspect('equal')

[TomAugspurger]

Added to #8312