Issue title: OnBlur not working
 Issue body: Why onblur and onfocus are fired together?

$('#teste').find('select').multipleSelect({
            onOpen: function() {
                alert('Select opened!');
            },
            onClose: function() {
                alert('Select closed!');
            },
            onCheckAll: function() {
                alert('Check all clicked!');
            },
            onUncheckAll: function() {
                alert('Uncheck all clicked!');
            },
            onFocus: function() {
                alert('focus!');
            },
            onBlur: function() {
                alert('blur!');
            }
        });

 Comments: 
 Comment 0: Because you use `alert`, so when you on focus it always on blur.
 Comment 1: I think this is not a bug.
{'source': 'GithubIssues', 'repo_name_with_owner': 'wenzhixin/multiple-select', 'issue_id': 106116589, 'repo_stars': 1871.0}