Adding datetime.timedelta to Series breaks via .add() but works via addition
Adding a datetime.timedelta to a series of datetime64[ns, UTC] breaks when using the Series .add(offset) method, but works when doing series + offset. This behaviour seems unexpected to me.

Furthermore, adding a pd.Timedelta drops timezone.

``` python
import pandas as pd
import datetime
import pytz

datetime_series = pd.Series([datetime.datetime(2016, 5, 1, tzinfo = pytz.utc)])
offset = datetime.timedelta(days=1)
print datetime_series.dtype
>>> datetime64[ns, UTC]

# Fails
datetime_series.add(offset)

----------------------------------------------------

---------
TypeError                   Traceback (most recent call last)
<ipython-input-12-89cc70a4801f> in <module>()
      8 
      9 # Fails
---> 10 datetime_series.add(offset)
     11 
     12 # Works as expected

/home/adrian/.virtualenvs/pandas/local/lib/python2.7/site-packages/pandas/core/ops.pyc in flex_wrapper(self, other, level, fill_value, axis)
    937                 self = self.fillna(fill_value)
    938 
--> 939             return self._constructor(op(self.values, other),
    940                                      self.index).__finalize__(self)
    941 

TypeError: ufunc add cannot use operands with types dtype('<M8[ns]') and `dtype('O')
```
#### Expected Output

``` python
# Works as expected
datetime_series + offset` 

>>> 0   2016-05-02 00:00:00+00:00
>>> dtype: datetime64[ns, UTC]
```

Moreover, if the usages of `offset` in all of the above code are replaced with `pd.Timedelta(offset)` then the breaking line works but drops timezone. The line described as working does not drop timezone when replacing with `pd.Timedelta(offset)`
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.19.0-66-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 26.0.0
Cython: None
numpy: 1.9.0
scipy: 0.14.0
statsmodels: None
xarray: None
IPython: 5.0.0
sphinx: None
patsy: None
dateutil: 2.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.4.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: 3.6.4
bs4: None
html5lib: 0.999
httplib2: 0.9
apiclient: 1.1
sqlalchemy: None
pymysql: None
psycopg2: 2.6.1 (dt dec pq3 ext lo64)
jinja2: 2.8
boto: 2.26.0
pandas_datareader: None
