Replace is_file() with the faster file_exists()
(where it makes sense)

Also:
 - Implemented caching of configuration arrays for smileys, foreign characters and doctypes.
 - Implemented cascading-style loading of configuration files (except for library configs, DB and constants.php).
Merge branch 'develop' of github.com:EllisLab/CodeIgniter into develop
[ci skip] Add some changelog entries
Replace CI_Upload::clean_file_name() usage with CI_Security::sanitize_filename()

Also applied @xeptor's fix (a big thanks) to the sanitize_filename() method and added a changelog entry for it - fixes issue #73.
fixed #2207 user guide error

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Merge pull request #2208 from appleboy/userguide

fixed #2207 user guide error