Added 404 Not Found page

app.rb

@@ -65,3 +65,7 @@ end
 get '/aliens' do
   haml :aliens
 end
+
+not_found do
+  haml :not_found
+end

views/not_found.haml

@@ -0,0 +1,7 @@
+.grid_9
+  %h1 Not Found
+  
+  %p Oops, we can't find that page.
+  
+  %p
+    %a{ :href => '/' }Go back to the home page