Added autoload part to composer.json for easiest autoloading

With this change, Composer autoloading will be easier.

With following structure (for example):
```
/protected
/vendor
/web/index.php
```

my `index.php` can look like this:
```php
$config=dirname(__FILE__).'/../protected/config/main.php';

// remove the following lines when in production mode
defined('YII_DEBUG') or define('YII_DEBUG',true);
// specify how many levels of call stack should be shown in each log message
defined('YII_TRACE_LEVEL') or define('YII_TRACE_LEVEL',3);

// register composer auto loader
require_once(dirname(__FILE__).'/../vendor/autoload.php');

Yii::createWebApplication($config)->run();
```

Because in `/vendor/composer/autoload_files.php` I have:
```php
<?php

// autoload_files.php @generated by Composer

$vendorDir = dirname(dirname(__FILE__));
$baseDir = dirname($vendorDir);

return array(
    $vendorDir . '/yiisoft/yii/framework/yii.php',
);

```

What do you think?