minimum-stability is only usable in a root composer.json

See http://getcomposer.org/doc/04-schema.md#minimum-stability
Add magento-composer-installer dependency

Without this dependency this module won't be installable via composer
Change extras field to extra

The field is called extra, not extras, according to
http://getcomposer.org/doc/04-schema.md#extra and to
./composer.phar validate