DOC: add a section on growing pandas ecosystem

[ghost]

Incresingly, python packages are being built on top of pandas.
Often, these solve problems pandas users need addressed
but which are a stretch for pandas' scope proper.
#2302, and pandas-xlsxwriter-charts, for example. vincent, yhat/ggplot, seaborn, etc'.

statsmodels too ofcourse, though it should be fairly well known.

Let's give them some exposure, encourage people to do their
own thing on top of pandas and help users locate existing solutions.

Any more mature(-ish) packages which deserve a mention?

[jreback]

geopandas - good example of sub classing frames

maybe add on to related library page?

[jreback]

prob need some vetting

https://github.com/paulgb/sklearn-pandas

http://wrobstory.github.io/2013/10/mapping-data-python.html

http://2013-cse801.readthedocs.org/en/latest/day9.html

http://blog.yhathq.com/posts/R-and-pandas-and-what-ive-learned-about-each.html

[ghost]

Go ahead and unleash your bookmarks. we'll sort through them later. :)

[jreback]

http://blog.yhathq.com/posts/pandasql-sql-for-pandas-dataframes.html