Authoring api exhibit

Two main changes in this pull request:
1. As per updated spec from MSU this change modifies the PUT exhibit API to allow a collection of content items to be passed in addition to the exhibit. The return value is now a list contains the guid of the exhibit and the guids of the new or modified content items.

2. Modified the Put requests for Timeline, Exhibit & ContentItem to use the existing entity classes and move away from using custom classes.