Review request for letting the application declare module metadata before the Loader is present

https://trello.com/card/my-yui-loader-should-honor-yui-env-version-in-order-to-attach-app-specific-meta-before-yui-meta/50f49c189b5810b542012e24/122

In order for an application to be able to declare module metadata without the Loader initializing first, we initialize `YUI.Env.[VERSION]` in the `yui` module and honor any existing module metadata by merging `YUI.Env.[VERSION]` instead of overwriting it.