Support for MIME types other than JSON

[kimburgess]

The current implementation is setup for working with JSON API's only. To provide more generalised use, it would be nice to implement the ability to register arbitrary parsers based on MIME type.

Potential syntax:

Responsible.parse "text/csv" do |response|
  CSV.new response.body || response.body_io
end

Sensible default can ship for out-of-the-box functionality as part of this.

[stakach]

seems like a good addition