Check whether $.fn.pickadate and $.fn.pickatime have been defined before set up translations

When I only include picker.js, picker.date.js, and translations/zh_CN.js, it produces an error: `TypeError: jQuery.fn.pickatime is undefined`.

It's better to check whether $.fn.pickadate and $.fn.pickatime have been defined before using them.

```
if (jQuery.fn.pickadate){
jQuery.extend( jQuery.fn.pickadate.defaults, {
});
}

if (jQuery.fn.pickatime){
jQuery.extend( jQuery.fn.pickatime.defaults, {
});
}
```

Or, we could set a default value to make sure they exists.

```
jQuery.fn.pickadate = jQuery.fn.pickadate || {};
jQuery.fn.pickatime = jQuery.fn.pickatime || {};
```