code: function gravatarUrl(email, size) {
      var url = 'http://www.gravatar.com/avatar/';
      if (email) {
          url += crypto.createHash('md5').update(email.toLowerCase()).digest('hex');
      }
      if (size) {
          url += '?s=' + size;
      }
      return url;
  }
docs: get gravatar url.
  
  @param {String} email
  @param {Number} [size]
  @return {String} gravatar url