ENH: Allow period_range to accept freq with mult != 1
`date_range` can accept freq with mult != 1 and output `DatetimeIndex`, otherwise `period_range` raises `ValueError`.

```
pd.date_range('2014-01-01 09:00', '2014-01-01 20:00', freq='4H')
# <class 'pandas.tseries.index.DatetimeIndex'>
# [2014-01-01 09:00:00, ..., 2014-01-01 21:00:00]
# Length: 4, Freq: 4H, Timezone: None

pd.date_range('2014-01-01 20:00', '2014-01-01 09:00', freq='-1H')
# <class 'pandas.tseries.index.DatetimeIndex'>
# [2014-01-01 20:00:00, ..., 2014-01-01 09:00:00]
# Length: 12, Freq: -1H, Timezone: None

pd.period_range('2014-01-01 09:00', '2014-01-01 20:00', freq='4H')
# ValueError: Only mult == 1 supported
```

Is it reasonable to accept `period_range` and `PeriodIndex.__new__` to this kind of freq? What I want to do is creating skipped or reversed ordered `PeriodIndex` easily. 
(Considering internal representation of `PeriodIndex`, it looks better to result have normal freq,  not to add numberized-freq like `4H`).
#### Expected

```
pd.period_range('2014-01-01 09:00', '2014-01-01 20:00', freq='4H')
# <class 'pandas.tseries.period.PeriodIndex'>
# [2014-01-01 09:00:00, ..., 2014-01-01 21:00:00]
# Length: 4, Freq: H

pd.period_range('2014-01-01 20:00', '2014-01-01 09:00', freq='-1H')
# <class 'pandas.tseries.period.PeriodIndex'>
# [2014-01-01 20:00:00, ..., 2014-01-01 09:00:00]
# Length: 12, Freq: H
```
