Use valid composer.json instead of non-standard extension.json

We should simply use the composer.json format and also call the file that? It seems like our current file is pretty much that anyway. A specifiation of the format, that already exists then: http://getcomposer.org/doc/04-schema.md

I've also created an issue for allowing the definition of a role for authors: https://github.com/composer/composer/issues/581

Discussion on Area51: http://area51.phpbb.com/phpBB/viewtopic.php?f=84&t=42567&p=237865#p237865