Fetch application description property and Fetch Item references

Adding property  `appDescription` in `PKTApp` class.
Adding `fetchItemReferencesWithID` method in `PKTItem` class.
Just a minor comment on the naming, to be consistent I would call it `+requestForReferencesForItemWithID:` instead, because its indicating that it is the ID of the item you are passing in, not the id of the references.