Add negative attribute reader and writer methods

This change adds negative attribute reader and writer methods like `Spaceship#not_warpdrive?` and `Spaceship#not_warpdrive=`. These methods will make integration with form builders such as `simple_form` and `formtastic` easier in certain situations.

Suppose you're using `simple_form` to implement a UI that requires a checkbox with the label "Warpdrive Off". RIght now, you would have to something like this:

```ruby
f.input :warpdrive, as: :boolean, label: 'Warpdrive Off', checked_value: '0', unchecked_value: '1'
```

These negative attribute methods would allow you to do something like this instead:

```ruby
f.input :not_warpdrive, as: :boolean, label: 'Warpdrive Off'
```

I also DRY'ed up the code a bit by using aliases to define predicate methods.