BUG: adding DatetimeIndex with series of timedeltas results in strange series
```
In [21]: dtidx = pd.date_range('2012-01-01', periods=2)

In [22]: dtidx
Out[22]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2012-01-01, 2012-01-02]
Length: 2, Freq: D, Timezone: None

In [23]: dtidx + pd.Series([pd.Timedelta('10 days'), pd.Timedelta('20 days')])
Out[23]:
2012-01-01 00:00:00   NaT
2012-01-02 00:00:00   NaT
0                     NaT
1                     NaT
dtype: datetime64[ns]
```
