Improved description of seaborn
The pandas docs send a surprising amount of traffic towards seaborn, but
I've never thought that the existing description communicates the
goals/advantages of seaborn all that well. Please consider this revised
copy.

I also swapped the ordering of seaborn/py-ggplot, because it looks like
that package has stalled out a bit, but I'm happy to revert the
reordering if it bothers people.