Added logout button

Added logged_in helper to allow us to check whether a user is logged in
to show the appropriate button

app/controllers/application_controller.rb

@@ -1,4 +1,5 @@
 class ApplicationController < ActionController::Base
+  helper_method :current_user, :logged_in?
 
   def require_login
     redirect_to login_url unless current_user.present?

app/views/layouts/application.html.erb

@@ -33,7 +33,11 @@
           <div class="navbar-end">
             <div class="navbar-item">
               <div class="buttons">
+              <% if !logged_in? -%>
                 <%= link_to "Login", login_url, { :class => "button is-primary"} %>
+              <% else -%>
+               <%= link_to "Logout", logout_url, { :class => "button is-light"} %>  
+              <% end -%>
               </div>
             </div>
           </div>