Minor changes

Focused on making the following change
  
1.Link to user on articles_path page
2. reformat post time (time_ago_in_words)
3.add link to show page
4.grab root level domain(method on article model) => use URI class
5. restriction on registration if user is logged in
6. be logged in when user creates account
7. test methods logged_in? and current_user in application spec