datetime not properly converted from Series; timezone dropped
This bug also happens if you use the .convert_objects() method of DataFrame or Series.

To reproduce:

```
In [83]: import pytz

In [85]: d = datetime.datetime(2014, 9, 3, 2, 40, 16, tzinfo=pytz.utc)

In [88]: pd.to_datetime(pd.Series([d]))[0]
Out[88]: Timestamp('2014-09-03 02:40:16', tz=None)

In [89]: pd.to_datetime([d])[0]
Out[89]: Timestamp('2014-09-03 02:40:16+0000', tz='UTC')

In [91]: pd.__version__
Out[91]: '0.13.1'
```
