BUG: first() changes datetime64 data
I have a dataframe that contains a datetime64 column that seems to be losing some digits after a groupby.first(). I know that under the hood these are stored at nanosecond precision, but I need them to remain equal, since I'm merging back onto the original frame later on. Is this expected behavior?

``` python
import pandas as pd
from pandas import Timestamp

data = [
 {'a': 1,
  'dateCreated': Timestamp('2011-01-20 12:50:28.593448')},
 {'a': 1,
  'dateCreated': Timestamp('2011-01-15 12:50:28.502376')},
 {'a': 1,
  'dateCreated': Timestamp('2011-01-15 12:50:28.472790')},
 {'a': 1,
  'dateCreated': Timestamp('2011-01-15 12:50:28.445286')}]

df = pd.DataFrame(data)
```

Output is:

``` python
In [6]: df
Out[6]:
   a                dateCreated
0  1 2011-01-20 12:50:28.593448
1  1 2011-01-15 12:50:28.502376
2  1 2011-01-15 12:50:28.472790
3  1 2011-01-15 12:50:28.445286

In [7]: df.groupby('a').first()
Out[7]:
                    dateCreated
a
1 2011-01-20 12:50:28.593447936
```

When I compare the datetime64 in the first row to the datetime64 after the groupby.first(), the two are not equal.
