bugfixes for haversine formula

[pdtyreus]

I found some issues with this implementation while testing. You can reproduce by holding the longitude constant and varying the latitude. The formula in the current implementation always returns 0 for the distance.

        float refLat = 24.396308f;
        float refLng = -124.848974f;

        ZFHaversine *haversine = [[ZFHaversine alloc] initWithLatitude1:refLat longitude1:refLng latitude2:38.896376f longitude2:refLng];
        float yKm = [haversine kilos];

        haversine = [[ZFHaversine alloc] initWithLatitude1:refLat longitude1:refLng latitude2:refLat longitude2:-77.328688f];
        float xKm = [haversine kilos];

        haversine = [[ZFHaversine alloc] initWithLatitude1:refLat longitude1:refLng latitude2:38.896376f longitude2:-77.328688f];
        float km = [haversine kilos];

        NSLog(@"translate %f %f %f",yKm,xKm,km);

I updated the formula based on the wikipedia version https://en.wikipedia.org/wiki/Haversine_formula.

I assume since this has been untouched for a couple years the implementation provided is working fine for your use. But I figured I would offer to contribute back since I found the code handy.