Replaced explicit call to Yii::autoload() with call to autoloader stack

This allows usage of normal class names to be loaded by autoloaders
prepended to the Yii::autoloader()

Should be BC if there is only Yii autoloader and not cause much trouble
when other autoloaders are properly implemented. Works fine with
composer autoloader.

fixes #2777
Re-added call to Yii::autoload to import() to keep speed

import will check for classmap first before triggering autoloader stack.

issue #2777
guide fix about autoloading
Merge branch 'import-autoloading'

* import-autoloading:
  Re-added call to Yii::autoload to import() to keep speed
  Replaced explicit call to Yii::autoload() with call to autoloader stack