this commit changes the syntax of create_posts and adds support to japr
(changing the value of the variable display_path)

More in details:
- changed order of arguments of create_posts.
  now date is the first argument (to reflect the structure of filename)
- added an argument category to the posts

Other changes
- added tasks list_changes
- refactored the code of list_changes and post_changes

- added support for japr in url setting. Now if you use japr to manage
  assets, the rakefile changes the value of display_path to ensure all
  references to assets are correct