Do not show datepicker for readonly inputs

[kix]

Currently, calling $(el).datepicker() on a readonly input (<input type="text" readonly/>) enables users to modify its content. This patch disables the datepicker popup from showing when an <input/> is readonly.

[acrobat]

Thanks @kix!

[MuyBien]

This code will not work if the datepicker class is append to a div (as in this example with the component : http://eternicode.github.io/bootstrap-datepicker/?markup=component&format=&weekStart=&startDate=&endDate=&startView=0&minViewMode=0&todayBtn=false&clearBtn=false&language=en&orientation=auto&multidate=&multidateSeparator=&keyboardNavigation=on&forceParse=on#sandbox).
I came up with this code. I think it needs to be changed before merged but it works :

    var dateInput = this.isInput ? this.element : this.element.find("input");
    if (dateInput.attr('readonly'))
        return;

I didn't modify the this.element because it will not work in the case you click the component element (this.element = [])

[natsu90]

It should be disabled on disabled attribute, instead of readonly. Readonly is useful to disallow user to edit datepicker input directly; 02/26/201 is treated as a valid date.