New public function 'refresh'

Hi

In my project I need to update calendar events on fly. Thus, I wrote new function which update all calendar events.

```javascript
$calendar.cal( 'refresh', events );
```
For each event in 'events' arg:
1. If event exists in calendar then function update it
2. If event does not exist in calendar, then function add it
 Each event which exists in calendar and does not exist in arg 'events' is removed