BUG/API: to_timedelta unit-argument ignored for string input
So, I don't know if this is by design, but it sure confused me. `to_timedelta()` operates differently on the number `1000` compared to the string `'1000'`:

``` python
In[3]: pd.to_timedelta(1000, unit='ms')
Out[3]: Timedelta('0 days 00:00:01')
In[4]: pd.to_timedelta(1000.0, unit='ms')
Out[4]: Timedelta('0 days 00:00:01')
```

and

``` python
In[6]: pd.to_timedelta('1000ms')
Out[6]: Timedelta('0 days 00:00:01')
```

while

``` python
In[5]: pd.to_timedelta('1000', unit='ms')
Out[5]: Timedelta('0 days 00:00:00.000001')
```

I've tried my best to come up with a reason for why this behaviour would be desirable, but I can't think of any.
