Add original metadata author to XWikiDocument

A new concept of author has been introduced and the way authors are managed in XWikiDocument have been refactored to rely on a unique interface DocumentAuthors.

We now distinguish 4 different kind of authors information:

• the creator: there's no ambiguity it's the user who created the document
• the content author: it's the author who performed a change in the content of the document, by opposition with changing a metadata of the document such as an xobject, or an attachment. We distinguish it for security reason since the content of the document might contain scripts that are executed when viewing the document
• the effective metadata author: it's the author which has been used to save any change in the document, content or not. So changing the content of the document will update both this author, and the content author. However, updating an attachment of a document will only update this author, not the content author. 
• the original metadata author: it's finally the author you see displayed in the interface, it's most of the time the same as the effective metadata author but in some usecases it might be different. For example when a script wants to allow a user to perform a save without changing the effective author for security reasons, but wants to display the author who actually triggers the save.

Only the original metadata author has been introduced in this release, but all authors have been renamed and now relies on UserReference.