Add yearIso property

This time of year usually gives problems with dates. I have a project that works with week numbers.

$date = new \Carbon\Carbon('2014-12-30');
echo $date->weekOfYear .' '.$date->year;

Gives `1 2014`. So the correct usage (in this particular case) is `$date->format('o')`, which gives 2015.

Would it be good to give this a property on the date object? Something like `$date->yearIso` / `$date->IsoYear`? Even though it's not much shorter then `format->('o')` but it might make it more obvious to people with autocomplete etc.