Strange output from timedelta_range for integer start/end
Here I expected a TimedeltaIndex with 11 elements from 0 hours through 11 hours:

```
>>> pd.timedelta_range(0, 10, freq='H')
<class 'pandas.tseries.tdi.TimedeltaIndex'>
['0 days']
Length: 1, Freq: <Hour>
```

It appears that `freq` is taken to refer only to the step size, not the start/stop, and the unit defaults to `ns` like Timedelta itself? I think we should either:
1. add a separate `unit` argument -- but this is a subtle distinction
2. use `freq` to also imply the `unit`
3. raise on integer input (possibly in addition to 1?)

Similarly strange and somewhat inconsistent with the above example:

```
>>> pd.timedelta_range(0, 10, periods=100)
ValueError: Must specify two of start, end, or periods
```
