Extended DataReader to fetch Dividends and Stock Splits from Yahoo!.
- Added yahoo-actions data_source to DataReader;
- Added get_data_yahoo_actions that returns a DataFrame containing
  DIVIDEND and SPLIT corporate actions fetched from Yahoo! Finance;
- Added unit tests;
- Added documentation example.

this is a nice add, pls direct your PR to https://github.com/pydata/pandas-datareader

as that is how updated to the `io/data.py` will be handled going forward.

thanks!
