Improve application speed by using classMap for autoloader

I ran a benchmark including yii2 and yii1.1 with the following results: http://www.stuff.cebe.cc/php-bench/yii/

It turned out that autoloader was the main bottleneck as it took more then 50% of request time: ![before](http://www.stuff.cebe.cc/php-bench/yii2-call.png)

After adding classmap we improved performance significantly:
![after](http://www.stuff.cebe.cc/php-bench/yii2-cm.png)