Code Igniter troubleshootings

[benja-M-1]

Add some troubleshoutings on CodeIgniter configuration documentation:

• issues with exception management
• issues with redirects
• issues with 404

Add some insights on how to fix those issues

[benja-M-1]

With CodeIgniter 2.1 I just faced an issue with the routing.
As codeIgniter does not have a different front_controller for another environment, the routing is based on the REQUEST_URI and the SCRIPT_NAME. But with app_dev.php, they are the same which cause a bug in the resolution: CodeIgniter always matches the / controller.