Length/size of datetime groups cast as datetimes
```
In [7]: pandas.Series([datetime.datetime.utcnow()]*4).groupby([1, 1, 2, 2]).agg('size')
Out[7]: 
1    2
2    2
dtype: int64

In [8]: pandas.Series([datetime.datetime.utcnow()]*4).groupby([1, 1, 2, 2]).agg(len)
Out[8]: 
1   1970-01-01 00:00:00.000000002
2   1970-01-01 00:00:00.000000002
dtype: datetime64[ns]

In [9]: pandas.Series([datetime.datetime.utcnow()]*4).groupby([1, 1, 2, 2]).agg(numpy.size)
Out[9]: 
1   1970-01-01 00:00:00.000000002
2   1970-01-01 00:00:00.000000002
dtype: datetime64[ns]
```

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

<details>
commit: 3853fe6d2b884f186b93933ef53ff5e475c2d80c
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-59-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: None.None

pandas: 0.19.0+395.g3853fe6
nose: 1.3.7
pip: 8.1.2
setuptools: 22.0.0
Cython: 0.24
numpy: 1.11.0
scipy: 0.17.1
statsmodels: None
xarray: None
IPython: 4.2.0
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_datareader: None
</details>
