expose the runtime YUI instance to RS addons

This is made of the following changes:
* added `getAppConfig()` to `lib/store.js`
* rearranged the server startup to:
  * call new `getAppConfig()` to get YUI configuration
  * create YUI instance
  * create store, passing along the YUI instance
  * rest of server initialization as before
* update `Store.createStore()` to attach the passed YUI instances to the returned store object as `store.runtimeYUI`