Date picker fixes

- use user's timezone when asking for date and time
- automatically build jQueryUI DatePicker format starting from the php date format
- parse the received date string accordingly to the used format
Bugfix
Remove special case 'DATE_PICKER' from tests

We now have to use DateHelper->getJQueryUIDatePickerFormat()
Show date/time in current timezone
Merge pull request #1782 from mlocati/fix-jqueryuidatepicker

Date picker fixes