Idempotency - A middleware for idempotency

This is a middleware that implements an RFC draft for idempotency-key-header.

Example code:

idempotencyMiddleware := idempotency.New(idempotency.NewMemoryStorage(),
	func(idempotencyKey string, w http.ResponseWriter, r *http.Request) {
		req, err := _state.FindByKey(idempotencyKey)
		if err != nil {
			http.Error(w, "No such key", http.StatusBadRequest)
			return
		}

		// Return content previously returned by myHandler.
	})

http.Handle("/", idempotencyMiddleware.Verify(myHandler))