groupby() changes values of pandas.Timestamp (pandas 0.15.2)
```
import datetime as dt; import pandas as pd
d2 = dt.datetime(2015, 4, 2, 12, 23, 12, 567000)
df = pd.DataFrame([['Bar', d2]], columns=['name', 'date'])
df2 = df.sort('date').groupby('name').last().reset_index()
```

`df2` is

name    date
0   Bar 2015-04-02 12:23:12.567000064

i.e. 64 nanoseconds were added to the timestamp. The amounts added seem to be always the powers of 2, perhaps it is caused by some internal conversion to double?
