ENH: IO World Bank WDI
The World Bank's World Development Indicators (WDI) is a collection of nearly 8000 data series in country-year format. These data cover a wide range of topics (e.g. demography, economics), and they are freely available freely through the World Bank API.

http://data.worldbank.org/

This PR introduces a very simple io module to search and downloading data from the WDI. Usage example: http://nbviewer.ipython.org/4365089/ 

I don't know the best way to test features such as this one, so I may need a bit of help there. 

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 I noticed that lots of data series weren't downloading properly. I'm investigating. If there's interest on your side, I will reopen as soon as I figure out what's happening. 
 OK. It looks like the World Bank advertises more data series than they actually publish through the API. I don't see an easy way to distinguish between available and unavailable series in the search() function, short of pinging each series. 

Thus, I think we can consider that this function works as-is. I will contact the World Bank people to see what can be done to improve, but this should still be useful now. 
 thanks!
 @vincentarelbundock I added issues #4354, do provide docs for this (could be pretty simple, really just advertising), maybe in io.rst in the Data section
