Use interpolation='none' when ploting with matplotlib in the first example of a heatmap

I realise that this adds additional complexity to the very early stages of the Python session. However the present heatmap doesnt' represent the data very well since additional points are interpolated between the data points. One could argue that the matplotib default should be 'none' but that would be a major api break. 

In a more advanced example I would also suggest changing the colormap to something better than the default (Jet) but at this stage I think it is an unnecessary complication.