groupby.first() casts datetime64[ns] series to 'object' with 'long" elements
```
import numpy as np
import pandas as pd
df = pd.DataFrame([(1, 1351036800000000000), (2, 1351036800000000000)])
df[1] = df[1].view('M8[ns]')
print df[1].dtype
print df.groupby(0).first()[1].dtype
print type(df.groupby(0).first()[1][1])

datetime64[ns]
object
<type 'long'>
```
