Fix Dutch dateFormat

The Dutch dateFormat should be dd-mm-yy. At the moment a German dateFormat is defined.

See also:
- http://en.wikipedia.org/wiki/Date_and_time_notation_in_the_Netherlands
- https://code.google.com/p/jquery-ui/source/diff?spec=svn3982&r=3982&format=side&path=/trunk/ui/i18n/jquery.ui.datepicker-nl.js