Yii framework v1.1 has it's own autoloader and its autoloader throws
an error, when it can't find class. When this library initializes, it
register its own autoloader, but places it at the end of autoload
queue. So when I use library with Yii 1.1, I just face an error:
`include(PHPExcel_Shared_String.php): failed to open stream: No such file or directory`
because Yii's autoloader runs first and tries to include class file
directly.
Registering our autoloader first avoid this.
