Incorrect result when using to_datetime() on PeriodIndex
Pandas 0.8.1:

```
import pandas as pd
index = pd.period_range('1/1/2012',periods=4,freq='D')
index.to_datetime()
```

returns incorrectly:

```
<class 'pandas.tseries.index.DatetimeIndex'>
[1970-01-01 00:00:00.000015340, ..., 1970-01-01 00:00:00.000015343]
Length: 4, Freq: None, Timezone: None
```

The correct result is obtained though with to_timestamp() instead of to_datetime():

```
index.to_timestamp()
```

returns correctly:

```
<class 'pandas.tseries.index.DatetimeIndex'>
[2012-01-01 00:00:00, ..., 2012-01-04 00:00:00]
Length: 4, Freq: D, Timezone: None
```
