function defineBooleanAttribute (key, handler) {
    var attr = $attrs.$normalize('md-' + key);
    if (handler) defineProperty(key, handler, undefined);
    if ($attrs.hasOwnProperty(attr)) updateValue($attrs[attr]);
    $attrs.$observe(attr, updateValue);
    function updateValue (newValue) {
      ctrl[ key ] = newValue !== 'false';
    }
  }
Defines boolean attributes with default value set to true. I.e. md-stretch-tabs with no value
will be treated as being truthy.
@param {string} key
@param {Function=} handler