Database model update

I put some modifications to the database model : 
     # tag_name is now the primary key of Tags
     # Users and Comments are linked now
     # add Groups (allow sharing betweens users)
     # add Privacies (for adjust privacy of an element)

With modification on Tags model, it is no enable to delete or update a tag throught the API. It is always possible to update tags for an element.

I add first routes and tests for Comments, Groups and Privacies.