Adding graceful shutdown of the repository with the application

Without a  `onStop` hook, the repository was reliant on the JVM terminating to shutdown.

Unfortunately, that's not the way SBT (and Play on top of it) reloads the running application in development mode - it reuses the same classloader, leading to warnings like "[The repository home ... appears to be in use since the file named .lock is already locked by the current process](http://wiki.apache.org/jackrabbit/RepositoryLock)".

The implemented `onStop`:

 * Uses reflection to find a `shutdown` command for the repository, and invokes it if found.
 * Sets `DIRECTOR` to `null` again so that it is reinitialized by the next `Application`.