Remove a function_exists() check for error_get_last()

It was only relevant until we dropped support for PHP < 5.2
Update CodeIgniter.php

require(APPPATH.'config/constants.php') should be in front of require(BASEPATH.'core/Common.php') because Common.php uses some constants defined in constants.php.
Small docblock fix
Merge pull request #2733 from vlakoff/develop

Small docblock fix
Merge pull request #2731 from josephok/develop

Update CodeIgniter.php
Always load application/config/constants.php