Add UnauthenticatedRouteMixin

This adds a mixin to redirect if the session is already authenticated.

I wasn't able to make sure this works in a real app because I'm using ember-cli-simple-auth, but all the tests pass.
How about `routeIfAuthenticated`, or `isAuthenticatedRoute`?  The latter resonates with the `session.isAuthenticated` proprtey.

As for `alreadyAuthenticatedRoute: this.routeAfterAuthentication` - this will result in `alreadyAuthenticatedRoute` being set to undefined as at the time of object declaration the this doesn't really exist.