Implemented the /thumbnail/PACKAGE url

Returns a 160x120 thumbnail image if possible
Returns a dummy image and a 404 status code otherwise.

config/routes.rb

@@ -19,6 +19,7 @@ Debshots::Application.routes.draw do
   post 'upload_image/:name' => 'packages#upload_image', as: :upload_image
   get 'delete_screenshot/:name/:id' => 'packages#delete_screenshot', as: :delete_screenshot
   get 'about' => 'welcome#about'
+  get 'thumbnail/:name' => 'packages#thumbnail', as: :thumbnail_image
 
   # The priority is based upon order of creation: first created -> highest priority.
   # See how all your routes lay out with "rake routes".