API: DatetimeIndex and PeriodIndex have same representation

Closes #7601. Made ``PeriodIndex`` to have dummy timezone representation to be consistent with ``DatetimeIndex``, but this should be ommited?

### Output after the fix
```
# DatetimeIndex (No change)
pd.date_range(start='2011-01-01', periods=10, freq='M')
# <class 'pandas.tseries.index.DatetimeIndex'>
# [2011-01-31, ..., 2011-10-31]
# Length: 10, Freq: M, Timezone: None

# PeriodIndex
pd.period_range(start='2011-01-01', periods=10, freq='M')
# <class 'pandas.tseries.period.PeriodIndex'>
# [2011-01, ..., 2011-10]
# Length: 10, Freq: M
```