API: .snap, xref Second().rollback(ts), rollforward(ts), onOffset(ts) does not change the timestamp
for example:

``` python
Second().rollback(pd.Timestamp('2015-02-02 08:35:22.555'))
```

does not change the timestamp and returns:

``` python
Timestamp('2015-02-02 08:35:22.555000')
```

This will probably also affect `DatetimeIndex.snap()` method.
(using pandas 0.17.1)
