Add a way to determine if it's a newly expressed emotions or not

I needed to be able to know if an emotion was newly expressed in ordre to launch a callback. As of today, the `express!` method was returning an `Emotion` instance with no clue of its state.

```RUBY
User.first.express!(:like, Post.first).new_record?
# => false (wrong: should be true)

User.first.express!(:like, Post.first).new_record?
# => false (right)
```

So, to achieve my goal, I've added a `newly_expressed?` method on the `Emotion` instance.

```RUBY
User.first.express!(:like, Post.first).newly_expressed?
# => true

User.first.express!(:like, Post.first).newly_expressed?
# => false
```

I'm not sure if it's the optimal way to achieve this though. Feedback on implementation will be greatly appreciated.