Feature: Add ability to raise errors from the responses

A client can now call raise_errors to raise errors on 5xx and 4xx response
codes.

    class Client < Rapidash::Client
    end

    client.some_invalid_resource #returns nil

    class Client < Rapidash::Client
      raise_errors
    end

    client.some_invalid_resource #Raises a Rapidash::ResponseError

Duplicate Response parsing code has been removed from the OAuthClient
and it now returns a Rapidash::Response object to be more in line with
the HTTPClient.