Allow specifying a default handler for all codes

If `default_handler` or `handler` is called with only a block and
without a status code passed, it will add a handler that will be called
for all status codes that don't have their own handler specified:

```ruby
class DropletKit < ResourceKit::Resource
  default_handler { 'Something unexpected happened!' }

  action :all, 'GET /droplets' do
    handler(:ok) { |response| Droplet.from_json(response) }
  end
end
```

If anything but a 200 is returned from `GET /droplets`, the specified
string will be returned. This fixes #7

Signed-off-by: David Celis <me@davidcel.is>
Remove dependency on ActiveSupport (fixes #8)

We only use ActiveSupport for a few core extensions that are easily
swapped out for their slightly-less-readable alternatives. There is also
a single instance of `class_attribute`, but that is easily implemented
as a mixin (included, tested)

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #10 from davidcelis/remove-activesupport

Remove dependency on ActiveSupport (fixes #8)
Merge pull request #9 from davidcelis/default-handler

Allow specifying a default handler for all codes