Add weeksInYear and isoWeeksInYear

[ichernev]

This implements #1289. I think the week and iso week stuff should go in
a separate calendar at some point.

[mdxs]

The following test could be added to test/moment/weeks_in_year.js to ensure a full coverage of the new isoWeeksInYear function in the current 2000 based cycle of 400 years:

    "count years with 53 iso weeks in year" : function (test) {
        // Based on http://en.wikipedia.org/wiki/ISO_week_date (as seen on 2014-01-06)
        // stating that there are 71 years in a 400-year cycle that have 53 weeks;
        // in this case reflecting the 2000 based cycle
        var count = 0, i, wks, yr;
        for (i = 0; i < 400; i++) {
            yr = 2000 + i;
            wks = (moment.utc([yr, 11, 31]).isoWeek() === 53) ? 53 : 52;
            count += (wks === 53) ? 1 : 0;
            test.equal(moment([yr]).isoWeeksInYear(), wks, yr + " should have " + wks + " iso weeks");
        }
        test.equal(count, 71, "Should have 71 years in 400-year cycle with 53 iso weeks");

        test.done();
    }

Based on one liner implementation for ISO as discussed in #1289 and on Wikipedia page referenced (see comments in above proposed additional test; note that there are similar/related tests in test/moment/weeks.js).

From local testing I can confirm that your implementation passes the above test, so 👍

[ichernev]

This is harder to read and I think the additional tests are more likely to fail in a browser with bad date support. So I'm against. @icambron can decide.

[icambron]

Yeah, I don't trust browsers to pass that test consistently.

[mdxs]

No problem.