VIS/DOC: clean-up plots in the docs
This is of course taste, but I personally find some of the plots on the docs not very pleasing to the eyes.
Some different things:
- [x] Too dense minor gridlines (solved by using mpl 1.4):
  - Eg the first plot on the page: http://pandas-docs.github.io/pandas-docs-travis/visualization.html#basic-plotting-plot
  - It becomes worse when the plots become smaller: http://pandas-docs.github.io/pandas-docs-travis/visualization.html#using-layout-and-targetting-multiple-axes
  - This could be solved using the new matplotlib styles or adapting our own `options.display.mpl_default` style
- [x] Overlapping axis labels (see also comment from here: https://github.com/pydata/pandas/pull/8301#issuecomment-56084391) (https://github.com/pydata/pandas/pull/8391):
  - See the examples with multiple axes: http://pandas-docs.github.io/pandas-docs-travis/visualization.html#using-layout-and-targetting-multiple-axes -> a lot of overlapping axis labels
- [x] Use square pie plots (https://github.com/pydata/pandas/pull/8391)
-  ...
