added a look up for nested presenters

[dspaeth-faber]

When you expose a property with a presenter like this:

expose :links, using: Link, :documentation => { type: 'array', items: {'$ref' => 'link'} }

The Link entity does not apear in the documentation unless you use it direktly for another action.
With this PR directly used presenters (not recursivly) are looked up and used.

Furthor more I did change parse_entity_name to use instance_variable_get(:@root)
because the property name must match my $ref entry. When you dont change this
the shown model name can differ from the linked one. Example:

module API
  class Link < Grape::Entity
      root 'links', 'link'
  end
end

Would be displayed as "link" for the Modelname, but I have to define

expose :links, using: Link, :documentation => { type: 'array', items: {'$ref' => 'API::Link'} }

And swagger would display the property like this:

links (array[API::Link], optional)

[dblock]

I like it. This needs tests and an update to CHANGELOG to be merged, please.

[dspaeth-faber]

@dblock I did add some spec's but merge with master will result in many merge conflicts.

[dblock]

You have to do the rebase work, please. If the merge looks crazy, I usually just copy-paste the code into the right place(s) in a new branch.

[dspaeth-faber]

I hope that it is now in a better condition. If not please give me a hint how I can improve the PR.

[dblock]

Looks great. Merged via tim-vandecasteele@860da15, thanks.

[dspaeth-faber]

Thank you too.