clear/reset select2，重置select2，恢复默认

4.0 version

//方法一
$('#yourButton').on('click', function() {
    $('#yourfirstSelect2').val(null).trigger("change");
    $('#yourSecondSelect2').val(null).trigger("change");
    ....
});

//方法二
$('#yourButton').on('click', function() {
    $('#yourfirstSelect2').select2('val','all');
    $('#yourSecondSelect2').select2('val','all');
    ....
});

3.0 version

$('#yourButton').on('click', function() {
    $('#yourfirstSelect2').select2("val", "");
    $('#yourSecondSelect2').select2("val", "");
    ....
});

ie/google经测试均可用，其他未测试