Cannot create DatetimeIndex using Period
This doesn't work. Looks like `start` and `end` should try a bit harder to convert to timestamp? 

```
from datetime import datetime
pd.DatetimeIndex(start=pd.Period('1990q1'), end=datetime(1992, 12, 31), freq='Q')
```

This does.

```
pd.DatetimeIndex(start=pd.Period('1990q1').to_timestamp(), end=datetime(1992, 12, 31), freq='Q')
```
