pd.to_timedelta(single_string) returns a Series, which ruins broadcasting.
Currently

```
In [10]: pd.to_timedelta(Series(['00:03:37', '00:05:05'])) - pd.to_timedelta('00:03:00')
Out[10]: 
0   00:00:37
1        NaT
dtype: timedelta64[ns]
```

because

```
In [13]: pd.to_timedelta('00:03:00')
Out[13]: 
0   00:03:00
dtype: timedelta64[ns]
```

More expected and consistent behavior, in my opinion, would be

```
In [12]: pd.to_timedelta(Series(['00:03:37', '00:05:05'])) - pd.to_timedelta('00:03:00')
Out[12]: 
0   00:00:37
1   00:02:05
dtype: timedelta64[ns]
```

If, instead, `pd.to_timedelta('00:03:00')` gave `numpy.timedelta64(180000000000,'ns')`, it would broadcast properly, as demonstrated by this work-around

```
In [12]: pd.to_timedelta(Series(['00:03:37', '00:05:05'])) - pd.to_timedelta('00:03:00')[0]
Out[12]: 
0   00:00:37
1   00:02:05
dtype: timedelta64[ns]
```

I have not regularly used timedeltas until now. Am I misjudging the expected usage?

**Update**, OK, one can also get the expected result by setting `box=False`. But `box=False` should be default for a scalar, no?
