Added the dayClass option

### Docs

The `dayClass` option allows to selectively specify which days on the calendar are enabled or not (at least this is what I need it for, though it can be used for other purposes as well).
If given then its value can be either:
  - a `String` - it is appended to the list of classes for each day on the calendar
  - a `Function` - the function is called with one parameter with details about the day being rendered:

```javascript
{
  cname:<String> a list of all classes determined to be applied for this day,
  row:<Number> the row index that this day is in in the calendar,
  col:<Number> the column index that this day is in in the calendar,
  calendar:<Object> (readonly) a 2D array of `moment` objects used to build the calendar (do not mess with this object as it is being used internally)
}
```

The function is expected to either return nothing (not even a blank string '') or return a string with the classes to be applied. The returned string can be anything you may need.

### Example

With this code you can add the class name `christmass` to Dec 25th of every year:

```javascript
$('#pickerElement').daterangepicker({
      dayClass:function (props){
         var d=props.calendar[props.row][props.col]
         if(d.date()==25 && d.month()==11)
            return props.cname+' christmass';
      }
   });
```

Then of course you need to add the required CSS declarations for `.christmass` to make it stand out in your calendar.