BUG: groupby.transform ValueError: Could not convert object to NumPy datetime
#### Problem description

As can be seen [here](http://stackoverflow.com/questions/42260764/pandas-timestamp-difference-in-groupby-transform), the error `ValueError: Could not convert object to NumPy datetime` is being thrown by:

```python
df = DataFrame(dict(A=Timestamp('20130101'), B=np.arange(5)))
result = df.groupby('A')['A'].transform(lambda x: x.max() - x.min())
```

NOTE: I will be submitting a PR shortly.

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little

pandas: 0.19.2
nose: 1.3.7
pip: 8.1.2
setuptools: 18.2
Cython: None
numpy: 1.12.0
dateutil: 2.6.0
pytz: 2016.6.1
</details>