JBPM-2913 let duedate of process support expression

[rebody]

Hi Maciej,

I repack your patch and upload it to the JIRA.  I think this patch is good enough.  Could you checked it into the svn when you have time?  Thank you very much.

https://jira.jboss.org/browse/JBPM-2913

[swiderski.maciej]

code checked in to SVN and jira issue resolved