Disable day of the week is localization dependent

I'm not sure if this is intended, but I think it makes user's work much harder. You can disable several days of the week with an disable array:

```
$('.datepicker').pickadate({
disable: [
1, 4, 7
]
});
```

Unfortunately 1 refers to the first shown day of the calendar, which can be Sunday as well as Monday, depending on the currently selected localization/firstDay setting. In my opinion it would be much easier if the the disable numbers refer to fixed days.