function _generateId(el) {
    var str = el.tagName + el.className + el.src + el.href + el.textContent,
      i = str.length,
      sum = 0;
    while (i--) {
      sum += str.charCodeAt(i);
    }
    return sum.toString(36);
  }
Generate id
@param   {HTMLElement} el
@returns {String}
@private