repr for PeriodIndex does not handle <=2 elements well
The `repr` for `PeriodIndex` uses ellipses (i.e. ...) for all number of elements. Ellipses should only be used for more than two elements. `DatetimeIndex` correctly does this:

```
In [9]: pd.period_range('2013Q1', periods=1, freq="Q")
Out[9]: 
<class 'pandas.tseries.period.PeriodIndex'>
freq: Q-DEC
[2013Q1, ..., 2013Q1]
length: 1

In [11]: pd.period_range('2013Q1', periods=2, freq="Q")
Out[11]: 
<class 'pandas.tseries.period.PeriodIndex'>
freq: Q-DEC
[2013Q1, ..., 2013Q2]
length: 2

In [10]: pd.period_range('2013Q1', periods=3, freq="Q")
Out[10]: 
<class 'pandas.tseries.period.PeriodIndex'>
freq: Q-DEC
[2013Q1, ..., 2013Q3]
length: 3
```

compare to:

```
In [16]: pd.date_range('2013-01-01', periods=3, freq="Q")
Out[16]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-03-31 00:00:00, ..., 2013-09-30 00:00:00]
Length: 3, Freq: Q-DEC, Timezone: None

In [13]: pd.date_range('2013-01-01', periods=2, freq="Q")
Out[13]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-03-31 00:00:00, 2013-06-30 00:00:00]
Length: 2, Freq: Q-DEC, Timezone: None

In [14]: pd.date_range('2013-01-01', periods=1, freq="Q")
Out[14]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-03-31 00:00:00]
Length: 1, Freq: Q-DEC, Timezone: None
```
