No prepend option for autoloaders

I had some issues with the fact that composer prepends his autoloaders. So I added an "no-prepend" option for both the install and update command. I believe this will be usefull for quite a few people out there. It will definitly be better than the hacky solutions suggested here https://github.com/composer/composer/commit/c80cb76b9b5082ecc3e5b53b1050f76bb27b127b#commitcomment-2427978 a while ago. :)