Critical Following the document:http://docs.moodle.org/en/Development:Libraries_Organization
1. Create a user/lib.php file
2. Create functions:
- user_create_user($user)
- user_delete_user($user)
- user_update_user($user)
- user_get_user($user)
3. Call these functions into user/externallib.php
Note: some comment in MDL-21226 can be helpful
