Commented Feature: DateTime type support [3]

Add `System.DateTime` type support for DKAL.
Add functions `date` to create DateTime's (via DateTime.Parse) and `now` to get current DateTime.
Support comparison operators on DateTime.
Comments: Example in http://dkal.codeplex.com/SourceControl/changeset/view/93ec18c3ea2d#ModularEngine/samples/MultiMain/datetime.mdkal