refactoring of gem to simplify client code, and allow the shutl_resource gem to not require a Quote.create(params, auth:token) every time
wip refactoring to allow for a simpler API and also only showing Shutl::Auth specific errors
passing specs for rescuing the Rack::OAuth2::Client::Error and raising a Shutl::Auth::InvalidCredentials error
added in some better exception handling and notification of Rack::OAuth exceptions