Pandas 0.18.1 df.groupby().count() throws "ValueError: Buffer has wrong number of dimensions" when one of the counted columns has dtype datetime64 
I've confirmed the error does not occur in Pandas 0.16 on a similar machine.
#### Code Sample, a copy-pastable example if possible

```
import pandas as pd
df = pd.DataFrame({'x': ['a', 'a', 'b'],
                   'y': [pd.Timestamp('2016-05-07 20:09:25+00:00'), pd.Timestamp('2016-05-07 20:09:29+00:00'), pd.Timestamp('2016-05-07 20:09:29+00:00')]})
df.groupby('x').count()
```
#### Observed output

---

```
ValueError                                Traceback (most recent call last)
<ipython-input-5-3119045de5b1> in <module>()
      2 df = pd.DataFrame({'x': ['a', 'a', 'b'],
      3                    'y': [pd.Timestamp('2016-05-07 20:09:25+00:00'), pd.Timestamp('2016-05-07 20:09:29+00:00'), pd.Timestamp('2016-05-07 20:09:29+00:00')]})
----> 4 print df.groupby('x').count()

/usr/local/lib/python2.7/dist-packages/pandas/core/groupby.pyc in count(self)
   3754         blk = map(make_block, map(counter, val), loc)
   3755 
-> 3756         return self._wrap_agged_blocks(data.items, list(blk))
   3757 
   3758 

pandas/lib.pyx in pandas.lib.count_level_2d (pandas/lib.c:23068)()

ValueError: Buffer has wrong number of dimensions (expected 2, got 1)
```
#### Expected Output

```
x  y
a  2
b  1
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-32-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.1
pip: 8.1.2
setuptools: 21.2.1
Cython: None
numpy: 1.11.0
scipy: 0.16.0
statsmodels: 0.5.0
xarray: None
IPython: 4.2.0
sphinx: None
patsy: 0.2.1
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: 0.8.0
tables: 3.1.1
numexpr: 2.2.2
matplotlib: 1.5.1
openpyxl: 1.7.0
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: 0.5.8
lxml: None
bs4: 4.2.1
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 0.9.7
pymysql: None
psycopg2: 2.5.3 (dt dec pq3 ext)
jinja2: 2.8
boto: 2.36.0
pandas_datareader: None
```
