Improve README and cli user feedback

- Don't add a todo that is an empty string, just abort instead.
- When adding a todo, display a message to say it was successfully added.
- Pressing spacebar will no longer throw an error in the delete todos when there are no todos to delete. Instead a message will be displayed saying there are no todos.
- logging saying all completed todos were deleted when they were.
- README fix.
This seemed to work ([demo](https://github.com/ryanseys/gcloud-node-todos/blob/rs-improve/README.md)).