Added function to check if data is restorable

Related to https://github.com/simsalabim/sisyphus/issues/92, enables easier confirmation before restoring:

    $('form.autosave').sisyphus({
        onBeforeRestore: function() {
            var self = this;
            
            if(self.hasDataToRestore()) {
                var confirmed = confirm("It looks like you closed the form without saving. Do you want to restore the previous values?");
                if( ! confirmed) {
                    self.manuallyReleaseData();
                }
                return confirmed;
            }
        }
    });