Fix Apple bug parsing fraction digits that round to next second.

The bug is documented in http://openradar.appspot.com/13024643,
http://openradar.appspot.com/12761015 and http://openradar.appspot.com/12029137

To workaround, we use a simple NSNumberFormatter to parse the fractional part,
and create the final result adding the intervals from the date without the
fractional part and the parsing result of the fraction part as a double.