Introduce "title" property to schema

Should fix everything but this part in the original issue: "If a "title" property exists, it is used as a label in listings. If it's empty, the package name could be used as a fallback ..."

Someone could please test the changes? phpunit somehow dies on me with a fatal error:
```
PHPUnit 3.7.15 by Sebastian Bergmann.

Configuration read from /var/www/composer/phpunit.xml.dist

...........................PHP Fatal error:  Call to a member function addAdapter() on a non-object in /var/www/composer/vendor/symfony/finder/Symfony/Component/Finder/Finder.php on line 70
```