Total seconds of pandas.NaT should become NaN
I see
```python
In[2]: pandas.tslib.Timedelta.total_seconds(pandas.NaT)
Out[2]: -9223372036.854776
```
#### Expected Output
```python
Out[2]: nan
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 25.1.6
Cython: 0.24.1
numpy: 1.11.1
scipy: 0.18.0
statsmodels: 0.8.0rc1
xarray: 0.8.2
IPython: 5.1.0
sphinx: 1.4.6
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: 1.4.1
bottleneck: 1.1.0
tables: 3.2.3
numexpr: 2.6.1
matplotlib: 1.5.2
openpyxl: 2.4.0
xlrd: 1.0.0
xlwt: None
xlsxwriter: 0.9.3
lxml: 3.6.4
bs4: 4.5.1
html5lib: 0.999999999
httplib2: None
apiclient: None
sqlalchemy: 1.0.14
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: 0.2.1
</details>
