Add error classes for new Twitter API v1.1 response codes

See https://dev.twitter.com/docs/error-codes-responses

lib/twitter/error/gateway_timeout.rb

@@ -0,0 +1,11 @@
+require 'twitter/error/server_error'
+
+module Twitter
+  class Error
+    # Raised when Twitter returns the HTTP status code 504
+    class GatewayTimeout < Twitter::Error::ServerError
+      HTTP_STATUS_CODE = 504
+      MESSAGE = "The Twitter servers are up, but the request couldn't be serviced due to some failure within our stack. Try again later."
+    end
+  end
+end

lib/twitter/error/rate_limited.rb

@@ -0,0 +1,10 @@
+require 'twitter/error/client_error'
+
+module Twitter
+  class Error
+    # Raised when Twitter returns the HTTP status code 429
+    class RateLimited < Twitter::Error::ClientError
+      HTTP_STATUS_CODE = 429
+    end
+  end
+end