BUG: PeriodIndex + offset incorrect
Troubleshooting based on `tests.indexes.period.test_tools.TestPeriodIndexConversion.test_to_timestamp_pi_combined`

```
idx = PeriodIndex(start='2011', periods=2, freq='1D1H', name='idx')

>>> idx + 1
PeriodIndex(['2011-01-02 01:00', '2011-01-03 02:00'], dtype='period[25H]', name=u'idx', freq='25H')

>>> idx + idx.freq
PeriodIndex(['2011-01-27 01:00', '2011-01-28 02:00'], dtype='period[25H]', name=u'idx', freq='25H')
```

`idx + idx.freq` should match `idx + 1`.