public ValidationResult ValidateRequest(AccessTokenRequest tokenRequest, NancyContext context)
        {
            var authorizationToken =
                this.authorizationTokenStore.Retrieve(context.CurrentUser.UserName);

            if (authorizationToken == null)
            {
                return ErrorType.InvalidGrant;
            }

            return ErrorType.None;
        }
        public AccessTokenResponse CreateAccessTokenResponse(AccessTokenRequest tokenRequest, NancyContext context)
        {
            this.authorizationTokenStore.Remove(context.CurrentUser.UserName);

            var token =
                string.Concat("access-token-", Guid.NewGuid().ToString("D"));

            this.accessTokenStore.Store(context.CurrentUser.UserName, token);

            return new AccessTokenResponse
            {
                Access_Token = token
            };
        }