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
```

doesn't need a tz
what I meant by consistent is the look/feel
 OK. Removed tz repr from `PeriodIndex`
 can this integrate some of the DatetimeIndex formatting stuff? and out in Base (while leaving the output the same)

in theory could do similarly with Series though might be much trickier 
 needs rebase
an u show the examples for both period and datetime in the top of PR
 Rebased and examples are attached.
 thanks!
