error messages can now be presented

This PR is a proposal (and misses specs & co) to present error messages like started to discuss [here](https://github.com/tim-vandecasteele/grape-swagger/pull/110).
It will be possible to do some thing like this:


```ruby

module  API
  module Entities
    class Error < Grape::Entity
      expose :error, documentation: { type: 'string', required: true }
      expose :code, documentation: { type: 'integer', required: true }
      expose :detail, documentation: { type: 'string', required: true }
      expose :href, documentation: { type: 'url', required: true }
      expose :rel, documentation: { type: 'string', required: true }
      expose :time, documentation: { type: 'datetime', required: true }

      def rel
        'self'
      end

      def href
        options[:env]['api.endpoint'].request.url
      end

      def time
        Time.now.utc
      end
    end
  end
end

error!({ error: 'Not Found',
            detail: "Some more information",
            with: API::Entities::Error
           }, 404)

```

This PR requires the upcomming version of  `grape-entity` .

I want to think about it. Can you remove it for now and split it into a separate issue? Sorry for the hassle.