[Issue #13] Add render syntactic sugar

As in issue #13 an example solution provided by @jrhe an implementation
of said feature has been created.

As per the discussion in the thread this is only a helper to be able to
reach the available options provided in the active_model_serializer gem.

usage is as follows:

```ruby
get '/some_path' do
  collection = Collection.all
  render collection, { meta: { current_page: 5 }, meta_key:
    :pagination_info }
end
```

The return value would be:
`{ pagination_info: { current_page: 5 }, collection: [item, item] }`

If given without a `meta_key` it would return as:
`{ meta: { current_page: 5 }, collection: [item, item] }`

Any feedback appreciated.

@zph, @olleolleolle and @bjoska
Merge pull request #15 from zph/issue_13

[Issue #13] Add render syntactic sugar