Improve repr for weekly Periods
It's totally possible I don't understand how to_timestamp should be used with a Period. But, I have a hunch there is a bug here.

```
In : p = Period('01-Jan-2012', 'W')
In : p.to_timestamp(how='E')
Out: <Timestamp: 2012-01-01 00:00:00>

In : p.to_timestamp('H', how='E')
Out: <Timestamp: 2012-01-01 23:00:00>
```

(found in 0.8b2)
