weekofyear gives different results on a DatetimeIndex (if tz is given)
Hello,

The following code

```
dates = ["2013/12/29", "2013/12/30", "2013/12/31"]
dates = pandas.DatetimeIndex(dates, tz="Europe/Brussels")
print dates.weekofyear
print [d.weekofyear for d in dates]
```

gives

```
[52  1  1]
[52, 52, 1]
```

i.e. calling weekofyear on the datetimeindex directly or iterating on the different dates of the datetimeindex gives different results.

Notice that if we remove the tz in the DatetimeIndex, the results are consistant.

Sébastien
