Fixed the extraction of the zip archive

The extracted Symfony folder is now longer kept inside the project
alongside a mirror of its content.
Closes #9
Merge branch 'fix_extraction' of https://github.com/stof/symfony-toolbelt into stof-fix_extraction

Conflicts:
	src/Symfony/Toolbelt/Installer/NewCommand.php
Merge branch 'stof-fix_extraction'