Add a 'hide.from.discovery' configuration option for remote IdP metadata. This allows to hide an IdP from the discovery service. Defaults to false, so every IdP in the metadata that doesn't have this option set to true will be listed.

Reformat SimpleSAML_Metadata_SAMLParser. Cleanup some unused properties. Removed unreachable statements.

Fix some more phpdoc issues with returned values. Add a getLastError() method to retrieve information about the last error occurred.