Unexpected result when slicing time series intra day
Time series slicing does not work intra day (Pandas 0.8.1):

```
import pandas as pd
import datetime as dt
rng = pd.date_range('1/1/2012', periods=4, freq='12H')
a = pd.Series(index=rng)
a[dt.datetime(2011,1,1,3):]
```

incorrectly results:

```
2012-01-01 00:00:00   NaN
2012-01-01 12:00:00   NaN
2012-01-02 00:00:00   NaN
2012-01-02 12:00:00   NaN
```

The same incorrect result is returned with:

```
a.ix[dt.datetime(2011,1,1,3):]
```

The correct result is obtained though with the truncate() method:

```
a.truncate(before=dt.datetime(2012,1,1,3))
```

correctly returns:

```
2012-01-01 12:00:00   NaN
2012-01-02 00:00:00   NaN
2012-01-02 12:00:00   NaN
```
