Monkey patching WorkPackage model to have extra display properties.

This doesn't feel nice, but I really didn't want to put model specific methods inside the element classes. Tried to put this code in an initializer but it complains about acts_as_journalized, I'm guessing something to do with the ordering of the includes.
Included display class in document generator. Not my proudest moment.
Merge pull request #18 from finnlabs/bug-4278-work-package-id-format

Monkey patching WorkPackage model to have extra display properties.