Create String setters for moment#month and moment#day

I wish to be able to set the month or weekday based on a name ('January' or 'Sunday') instead of a number (0). In other words, I want to use `moment().month('February')` or `moment().day('Sunday')`.

I should note that you can work around this limitation for month names by creating a second moment: `moment().month( moment('February', 'MMMM').month() )`.