Issue686 Collection validation for Timelines, Exhibits & ContentItems

Code review completed at http://mrccodereview.cloudapp.net/ui#review:id=1277

It is possible to create timelines, exhibits and content items across collections because the collection validation checks are missing.

For example, assume you are authoring in the sandbox collection. It is possible to add an exhibit with a parent timeline belong to a different collection, say Beta Content.

This fix adds checked to PutTimeline(), PutExhibit() and PutContentItems() to ensure that authoring cannot occur cross collection.

I tested timeline, exibhit and content item cases in fiddler with 1. Valid parent field 2. Parent field in a different collection from sandbox and 3. no parent field specified and validated we hit the new code check when necessary + entities are created only when valid.