add Representable::JSON::AutoCollection

this simplify the following:
```ruby
module SongRepresenter
  include Representable::JSON

  property :title
end

module SongsRepresenter
  include Representable::JSON::Collection

  items :class => Song
end

[Song.new(:title => "Fallout"), Song.new(:title => "Synchronicity")].extend(SongsRepresenter).to_json
```
to this:
```ruby
module SongsRepresenter
  include Representable::JSON::AutoCollection

  items :class => Song do
    property :title
  end
end

[Song.new(:title => "Fallout"), Song.new(:title => "Synchronicity")].extend(SongsRepresenter).to_json
```

The cases where you need a collection representer not DRY at the moment. Potentially, we can build it directly into Collection. The same should be follow as XML::Collection when #19 works