Edits README to include comment about raw_response.

* Adds comment on how to access and navigate `Response#raw_response`.
* Refactors `Marvelite::API::Client#build_response_object` method to
  delegate to `Response.make`. This way, the responsibility to select
  the correct `Response` object lies in the same `Response` class.
Merge pull request #11 from antillas21/raw

Edits README to include comment about raw_response.