Use -weekOfYear instead of -week to remove iOS 7 deprecation warnings.

Hey! I'm a big fan of DateTools but it generates 14 warnings when building against the iOS 7 / iOS 8 SDK because NSDateComponents -week has been deprecated in favor of -weekOfYear. -weekOfYear became available in iOS 5, so I suggest moving to that unless this library still needs to support iOS 4 (I hope not!)

I've confirmed that all the unit tests still pass after these changes, and the library no longer generates any warnings.