Add admin user

@rueckstiess, I'm taking a hack at #156.  This pull request probably isn't ready for merging, but could you take a look and let me know if it's going in a desirable direction?  The gist of changes is to start the standalone/replica set/sharded cluster w/o authentication, add a default admin user (w/ default roles), stop the standalone/replica set/sharded cluster, and restart the standalone/replica set/sharded cluster w/ authentication.