Yahoo URL has changed...
I believe the url for gathering data from yahoo has changed...

If i create a new DataReader... ticker_data = DataReader("SPY", "yahoo")

I get..

IOError: after 3 tries, Yahoo! did not return a 200 for url 'http://ichart.yahoo.com/table.csv?s=SPY&a=0&b=1&c=2010&d=11&e=31&f=2013&g=d&ignore=.csv'

I believe .finance needs to be added to the URL. (Ex) 'http://ichart.finance.yahoo.com/table.csv?s=SPY&a=0&b=1&c=2010&d=11&e=31&f=2013&g=d&ignore=.csv'
