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. 
