Add name validation in meta

[f3ath]

Need to validate attribute names in meta objects. The validation should be deep, e.g. in this example an exception should be thrown:

$m = Meta::fromArray([
  'valid_name' => [
    'valid_name_again' => [
      '#invalid_name_hidden_deeply' => 'omg'
    ]
  ]
]);

Add tests to test/MemberNamesTest.php.

[vtsykun]

Hi @f3ath
please review #67