Multi selection feature

I added these methods to support the multi selection
```java
public void init(Iterable<Date> selectedDates, Date minDate, Date maxDate);
public Iterable<Date> getSelectedDates();
```

I also added this init method without specifying a selected date :
```java
public void init(Date minDate, Date maxDate)
```
and in this case, we can set the multi selection behavior with this setter :
```java
public void setMultiSelect(boolean value)
```