Bottstrap 3 : Upgrade DatePicker to the latest version which is Bootstrap3 compliant.

I've upgrade the DatePicker with the latest code from : https://github.com/eternicode/bootstrap-datepicker
I've used the boostrap3 css version of course!

I've changed DateTextFieldConfig  so it is not final anymore and can be override.
Use case : 
``` Java
final DateTextField dateDebutField =
	        new DateTextField("startDate", new MyDateTextFieldConfig(getLocale()));
```
I've added a getConfig() in The DateTextField so we can get the config when overriding the class.