Performance decrease of groupby.first for datetime64 in 0.14
I have seen a dramatic (around 100x) slowdown of `groupby.first` method for `datetime64` type after updating to 0.14.

Here is an example:

``` python
from time import time
d = pd.DataFrame({'a': pd.date_range('1/1/2011', periods=100000, freq='s'), 'b': range(100000)})
t = time()
d.groupby('b').first()
print(time() - t)
```

Result:

```
3.87164497375
```

Same for `int` type

``` python
from time import time
d = pd.DataFrame({'a': range(100000), 'b': range(100000)})
t = time()
d.groupby('b').first()
print(time() - t)
```

Result:

```
0.0148799419403
```
