add Representable::JSON#collection_wrapper

this is a alternative way compare to #20 to make Collection Representers more DRY

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 SongRepresenter
  include Representable::JSON
  property :title
end

[Song.new(:title => "Fallout"), Song.new(:title => "Synchronicity")].extend(SongRepresenter.collection_wrapper(:class => Song)).to_json
```

Im not really sure what is the best method name is, #collection_wrapper or #collection_representer?

The same should be follow as XML::Collection when #19 works