to_timedelta does not recognize 'h' and 'm' units
'h' and 'm' units are not correctly recognized in to_timedelta:

```
print pd.to_timedelta(1,unit='D')
print pd.to_timedelta(1,unit='h')
print pd.to_timedelta(1,unit='m')
print pd.to_timedelta(1,unit='s')
print pd.to_timedelta(1,unit='ms')
print pd.to_timedelta(1,unit='us')
print pd.to_timedelta(1,unit='ns')
```

results in

```
86400000000000 nanoseconds
1 nanoseconds
1 nanoseconds
1000000000 nanoseconds
1000000 nanoseconds
1000 nanoseconds
1 nanoseconds
```

version: 0.14.0

Most likely related to Issue #6423
