BUG: period on weekends rolling to odd business days
For biz days, seems saturday is rolling to monday, but sunday to friday
looks like a bug (though it might just be ambiguous/undefined behavior).

```
In [11]: pd.Period('2013-10-4', freq='B')
Out[11]: Period('2013-10-04', 'B')

In [12]: pd.Period('2013-10-5', freq='B')
Out[12]: Period('2013-10-07', 'B')

In [13]: pd.Period('2013-10-6', freq='B')
Out[13]: Period('2013-10-04', 'B')

In [14]: pd.Period('2013-10-7', freq='B')
Out[14]: Period('2013-10-07', 'B')
```
