BUG: to_datetime ignores utc=True when arg is Series 
Is this a bug or a feature that I don't understand:

``` python
data = ['20100102 121314', '20100102 121315']
print (pd.to_datetime(data, format='%Y%m%d %H%M%S', 
                      utc=True)[0]).__repr__()
print (pd.to_datetime(pd.Series(data), format='%Y%m%d %H%M%S', 
                      utc=True)[0]).__repr__()
print (pd.to_datetime(pd.TimeSeries(data), format='%Y%m%d %H%M%S', 
                     utc=True)[0]).__repr__()
```

results in

``` bash
Timestamp('2010-01-02 12:13:14+0000', tz='UTC')
Timestamp('2010-01-02 12:13:14', tz=None)
Timestamp('2010-01-02 12:13:14', tz=None)
```

Using version '0.13.1'

---

Other example in https://github.com/pandas-dev/pandas/issues/15760