Poor weekofyear performance
```
In [14]: rng
Out[14]: 
<class 'pandas.tseries.index.DatetimeIndex'>
freq: <1 Day>, timezone: None
[1800-01-01 00:00:00, ..., 2000-01-01 00:00:00]
length: 73049

In [15]: timeit rng.weekofyear
1 loops, best of 3: 501 ms per loop

In [16]: timeit rng.year
100 loops, best of 3: 3.48 ms per loop
```
