Partial date indexing off by one error
```
In [47]: rets.ix['2000']
Out[47]:
<class 'pandas.core.frame.DataFrame'>
DatetimeIndex: 261 entries, 2000-01-03 00:00:00 to 2001-01-01 00:00:00
Data columns:
AAPL    261  non-null values
GOOG    261  non-null values
MSFT    261  non-null values
dtypes: float64(3)
```
