SimpleSAML namespace usage in config.php

Minor changes to use SimpleSAML namespace to Logger logging level definitions.

Take #2. Use the recently migrated SimpleSAML_Logger::maskErrors() and popErrorMask() methods to simplify code un SimpleSAML\Utils\Config\Metadata.

Move the error-reporting logic in SimpleSAML_Utilities to the SimpleSAML_Logger class, and add a new method there to evaluate whether an error would be masked or not according to its level.

RegistrationInfo can be inherited from EntitiesDescriptor

Expose RegistrationInfo in parsed metadata

- Add test case
- Fix .gitignore since it ignored all metadata folders

Add AttributeValueMap Authentication Processing Filter and unittest. 

Add a temporary autoloader that allows loading old classes (SimpleSAML_Path_Something) that have already migrated to namespaces (SimpleSAML\Path\Something), while logging a warning. This commit requires to regenerate the composer autoloader with ./composer.phar dump-autoload.

Make the module autoloader to register class aliases for the classes that are migrated to namespaces, so that they are still accessible via the old name. This keeps everything working while migrating the classes.