Allow Series to merge multiple values w/ the same datetime.
We are having a problem when loading daily historical dividend data into a pandas Series().  Because companies sometimes declare two different dividends on the same day, we have an issue when trying to index this series

The following

```
from pandas.core.api import Series
import datetime
data_seq = [.10, .23]
date_seq = [datetime.datetime(2011, 1, 1, 0, 0), datetime.datetime(2011, 1, 1, 0, 0)]
series = Series(data_seq, index=date_seq)
series[datetime.datetime(2011, 1, 1, 0, 0)]
```

Returns

```
Exception: Index values are not unique
```

We're hoping for a function that will go through a pandas a series and produce a new series w/o duplicate dates via some sort of combination function. In this case, it makes the most sense to combine the two dividends by adding them together.
