Use NSCalendarUnitWeekOfYear

Unfortunately I can't prove it with unit tests (see #34). But with old api I was getting incorrect result in the project I'm working on.

![2014-10-03_1148](https://cloud.githubusercontent.com/assets/217368/4503669/a3c2cc08-4ada-11e4-810b-3679900ece12.png)

And with the fix proposed here everything is good to me :)

![2014-10-03_1154](https://cloud.githubusercontent.com/assets/217368/4503680/078ecaa2-4adb-11e4-9419-ab6bceea43d6.png)