Enh #3533: Added Yii and YiiBase classes to composer autoloader.

I have chosen to add both YiiBase and Yii classes.

This will allow for a simple entry script like:
        require_once __DIR__ . '/vendor/autoload.php';
        $config = __DIR__ . '/config/web.php';
        Yii::createWebApplication($config)->run();

Note that the composer classmap generator allows overriding classes. So if I wanted to use a custom Yii class I'd just include something like this in the projects' composer.json:
        "autoload" : {
                "classmap" : [
                        "protected/components/Yii.php"
                ]
        ]

This would mean my own Yii classed is used instead of the placeholder provided by Yii.