Add name validation in meta

Need to validate attribute names in `meta` objects. The validation should be deep, e.g. in this example an exception should be thrown:

```php
$m = Meta::fromArray([
  'valid_name' => [
    'valid_name_again' => [
      '#invalid_name_hidden_deeply' => 'omg'
    ]
  ]
]);
```
Add tests to [test/MemberNamesTest.php](../tree/master/test/MemberNamesTest.php).