io.data._sanitize_dates behavior vs. docs
Docs say default start for DataReader should be 2010-1-1, but it returns 1 year ago from today. I prefer the docs. Thoughts?

```
>>> from pandas.io.data import _sanitize_dates
>>> _sanitize_dates(None, None)                                                               
(datetime.datetime(2011, 10, 3, 17, 32, 52, 969119),
 datetime.datetime(2012, 10, 2, 17, 32, 52, 969163))
```
