timedelta from string requires leading 0
http://stackoverflow.com/questions/29573441/calculate-difference-between-times-rows-in-dataframe-pandas#comment47297436_29573471

```
In [11]: pd.to_timedelta("06:40:00")
Out[11]: Timedelta('0 days 06:40:00')

In [12]: pd.to_timedelta("6:40:00")
ValueError: cannot create timedelta string converter for [6:40:00]
```
