PSR-0 Composer Autoloading

I see there is progress for PHPUnit to move towards the usage of Composer's management. 

I am recommending a switch from the custom autoloading into Composer's built in autoloading, as you are already using Composer for installing dependencies.

All we need to do is simply to merge the pull request and afterwards we can remove the unused `require_once` line in PHPUnit file `Autoload.php`.