Fix form/date_time helper

This fixes a few problems with the datetime helper when it comes to localization:

* The widget now uses one field for public display and one for storing the picked date as a timestamp. So the public date doesn't have to be parsable with strtotime(). The backend can work with the timestamp field. (Please note that javascript timestamps are ms and not s) 
* The initial value is generated using DATE_APP_DATE_PICKER. (As opposed to DATE_APP_GENERIC_MDY) So it won't differ from the picked value.
* The values for "AM" and "PM" are automatically displayed in the current locale using DateHelper:date()

Bug: http://www.concrete5.org/index.php?cID=547045