Added signin/signout logic, tests fails 'cuz I think I don't understand the session logic well
writing some tests concerning the post-user association
updated models
Change a little thing concerning a factory
So far every tests passes. I put one test pending because I've to use a factory to override the access restriction to micropost.created_at which is not possible as the factories doesn't work for an unknown reason (tested with attr_accessible :created_at inside the post model, but I rolled this back because that's ugly)
Getting back to the before each system: else the last post test wouldn't pass because both user and posts wouldn't be created. Added a commented code in relation with the fact the factory doesn't work
Destroy post logic