Add Objective-C specific type converters to QDateTime

This patch adds the Objective-C NSDate/CDateRef converters to
QDateTime

[ChangeLog][QtCore][Objective-C] Added NSDate/CDateRef converters for
QDateTime

Task-number: QTBUG-37116
Change-Id: I937ea927083a2767b5b17f10a48bf453c9ff8b01
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>