Incorrect unit conversion in `to_timedelta` when unit='h' or unit='m'
This issue was uncovered in this [Stackoverflow question](http://stackoverflow.com/q/25797245/190597):

Using Pandas 0.14.0: `pd.to_timedelta` seems to ignore the units when  `unit` is `h` or `m`:

```
In [263]: pd.to_timedelta(16, unit='h')
Out[263]: numpy.timedelta64(16,'ns')

In [264]: pd.to_timedelta(16, unit='m')
Out[264]: numpy.timedelta64(16,'ns')
```

Other units seem to work as expected:

```
In [269]: pd.to_timedelta(16, unit='D')
Out[269]: numpy.timedelta64(1382400000000000,'ns')

In [270]: pd.to_timedelta(16, unit='s')
Out[270]: numpy.timedelta64(16000000000,'ns')

In [271]: pd.to_timedelta(16, unit='ms')
Out[271]: numpy.timedelta64(16000000,'ns')

In [272]: pd.to_timedelta(16, unit='us')
Out[272]: numpy.timedelta64(16000,'ns')

In [273]: pd.to_timedelta(16, unit='ns')
Out[273]: numpy.timedelta64(16,'ns')
```

I think the behavior comes from `tslib.convert_to_timedelta`:

```
In [279]: import pandas.tslib

In [280]: pandas.tslib.convert_to_timedelta(16, 'h')
Out[280]: numpy.timedelta64(16,'ns')

In [281]: pandas.tslib.convert_to_timedelta(16, 'D')
Out[281]: numpy.timedelta64(1382400000000000,'ns')
```
