Add isoWeekDay to Weekday object

[gazben]

Currently the $weekday variable will contain the result of the array_search($weekday, $this->days);. The $days array starts with "MO" and not "SU" as stated in the comment above.

To use this library with Carbon I suggest to add the ISO weekday to the Weekday object.

The ISO weekday definition: https://en.wikipedia.org/wiki/ISO_week_date

Weekday number, a digit d from 1 through 7, beginning with Monday and ending with Sunday