Implement action verb aliases

I've implemented and documented HTTP verb methods which can be used instead of `#action`, like this:

```ruby
class DropletResource < ResourceKit::Resource
  resources do
    get :all, '/v2/droplets' do
      handler(:ok) { |response| DropletMapping.extract_collection(response.body, :read) }
    end
  end
end
```

These Sinatra-style declarations are much easier to read and understand. The implementation is simple however I'm not too sure as to how to write specs for this feature. Something like this maybe:

```ruby
(ResourceKit::ALLOWED_VERBS << :action).each do |verb|
  describe "##{verb}" do
    it 'yields an action to the block' do
      expect { |b| collection.send(verb, :all, &b) }.to yield_with_args(instance_of(ResourceKit::Action))
    end

    it 'adds the action to the collection' do
      action = collection.send(verb, :all)
      expect(collection).to include(action)
    end

    it 'accepts a second argument of VERB /resource' do
      action = collection.send(verb, :all, '/v2/droplets')
      expect(action.verb).to eq (verb == :action ? :get : verb)
      expect(action.path).to eq '/v2/droplets'
      expect(action.name).to eq :all
    end

    context 'when default handlers have been specified on the collection' do
      let(:handler) { proc { |_response| 'sure' } }

      before { collection.default_handler(:ok, &handler) }

      it 'prepends the default handlers to the test' do
        action = collection.send(verb, :all)
        expect(action.handlers[200]).to eq(handler)
      end
    end
  end
end
```

I've also added an example to `examples/` which uses this new syntax and doesn't require any authentication keys so it can be run by any user (unlike the existing Droplet example).

What do you think?
So for this, it's actually not needed if you're just returning the response body. That is the default behavior.