add test case for user-logged in
Hide "login" button when the user is logged in
1. make sure only logged_in user can access edit post page
2. make sure only the owner of the post can edit it
3. Remove the User_id field in post page
4. Hide "Create a post" button when user is not logged in
Merge branch 'master' into filter

Conflicts:
	app/models/post.rb