Grape Entity Support

Inspired by the on pull request #30 by @TLmaK0, this pull request adds support for generating swagger responseClass and models per the [Swagger API Declaration](https://github.com/wordnik/swagger-core/wiki/API-Declaration) based on Grape Entities.

Usage would look like the following 

```ruby
module API

  module Entities
    class Status < Grape::Entity
      expose :text, :documentation => { :type => "string", :desc => "Status update text." }
    end
  end

  class Statuses < Grape::API
    version 'v1'

    desc 'Statuses index', {
      :entity => API::Entities::Status
    }
    get '/statuses' do
      statuses = Status.all
      type = current_user.admin? ? :full : :default
      present statuses, with: API::Entities::Status, :type => type
    end
  end
end
```

And would generate the following swagger json

```json
{
  "apiVersion": "0.2",
  "swaggerVersion": "1.1",
  "basePath": [..],
  "resourcePath": "/statuses.{format}"
  
  ...

  "apis" : [...]
  "models": {
    "Status": {
      "id": "Status",
      "name" : "Status",
      "properties": {
         "text": {
           "type": "string",
            "desc": "Status update text."
         }
      }
    }
  }
```