Moved bootstrapping of plugin to plugins_loaded hook

This allows for checking if WordPress SEO Premium is running and if it is, not loading WordPress SEO files. Prevents a fatal error when activating premium while still running WP SEO.

This needs https://github.com/Yoast/wordpress-seo-premium/pull/95 to prevent the fatal error upon plugin activation of Premium.

**Note**
WordPress SEO hooks into `plugins_loaded` a couple of times more so the hook priority should not be set any higher than 14. WP SEO Premium can use the same hook for bootstrapping but with a hook priority of 1, to ensure it is loaded first.