Added moment.fn.(isoYear|(iso)?[Ww]eekday) and format tokens

Initial implementation of iso-week-year. Related to #613.

I'll add some tests, just let me know if I'm on the right track. If I'm not mistaken, the fact that iso-week-year can be different than the year should also mean that the localized week-year might be different than the year. In that case we need more functions/tokens :)