NaT treatment in pandas groupby/nunique
#### Code Sample, a copy-pastable example if possible

Calling nunique on a column that contains NaT results in this FutureWarning from numpy:

```
 python35\lib\site-packages\pandas\core\groupby.py:2854
 FutureWarning: In the future, NAT != NAT will be True rather than False.
  inc = np.r_[1, val[1:] != val[:-1]]
```

It might be worth updating the relevant lines in a way that does not depend on != comparison of NaT, otherwise `nunique()` behavior will unexpectedly change when [numpy is upgraded to 1.13](http://docs.scipy.org/doc/numpy-dev/release.html#future-changes).
#### Expected Output
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 26.1.1
Cython: 0.24.1
numpy: 1.11.0
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: None
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: 3.2.3
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.5
xlrd: None
xlwt: None
xlsxwriter: None
lxml: 3.6.4
bs4: None
html5lib: None
httplib2: 0.9.2
apiclient: 1.5.0
sqlalchemy: 1.0.14
pymysql: None
psycopg2: 2.6.2 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: None
pandas_datareader: None
