Upgrade minimal version of matplotlib to 1.5.3 and use a seaborn-like style in docs
Matplotlib is an optional dependency of pandas. I propose making matplotlib 1.5.3 the recommended minimal version for pandas 0.21. Currently, v1.4.3 is the minimal version.

The reason are:

* matplotlib v1.4 has support for python up to 3.4. it doesnt officially support python 3.5 or above. it's old.
* matplotlib 1.5 has support for python up to 3.5. it doesnt officially support python 3.6, but this is still better than for 1.4.
* matplotlib 1.5 has some new built-in styles, among others seaborn-like styles. IMO the ggplot-based plot that are used in the docs at present have a bit of  90-feel, while (IMO, this is subjective) the seaborn styles have a more modern feel. By upgrading the minimum version to 1.5 we could make the  plots in the docs switch style to seaborn-style.
* also, matplotlib 1.5 has built-in support for labeled data (a la pandas' column names). This upgraded requirement, while it may not affect users of pandas directly, could allow some refactoring of the plotting code in pandas (disclaimer, I haven't looked at the pandas plotting code).

For differences between the different 1.x versions for matplotlib, see [her](http://matplotlib.org/1.5.0/users/whats_new.html).

So:
* do people support making matplotlib 1.5 the minimum recomemnded version for pandas 0.21?
* Do people support switching the plot style in docs to a seaborn style?