refactoring: accidently mixed up model.blog and model.users

It was too late :/
blog: allow creating messages without being logged in
updated README
blog: fixed blog security settings

Security was switched off for the blog/create controller, but must
be anonymous to allow logged in users to create posts with their name.
blog: added blogEntry.creationDate
mongodb: fixed problem with blog entry id

The id must be null otherwise the MongoDB driver will refuse to set the id
generated by the database.
updated README