Adding empty? method to ListObject.

As a Rails developer I generally expect collections to respond to `empty?`. I was supervised when learning the Stripe API with stripe-ruby Gem that this did not work:

```
customer = Stripe::Customer.retrieve(stripe_customer_id)

if customer.cards.empty?
  # Make customer add card.
else
  customer.cards.each do |card|
    # Display card..
  end
end
```

I realize you can do: `customer.cards.count == 0` but that is not as clean in my opinion.