Composer tweaks

1. Moved autoloading of test classes to autoload-dev. This should be a no-brainer.
2. Made zendframework/zend-db an optional dependency. Not sure if you really want this. I will write a Doctrine adapter for my current project as it already uses Doctrine (i can contribute it back if you're interested), so i don't need Zend\Db. If you don't like this change, feel free to only cherry-pick the other commit.