Minor format and exception message fixes, plus add information about the different communication channels we have today.

bugfix: When obtaining a UserID for the state array prior to running authproc filters, if the source attribute had multiple values a warning was logged but the UserID was still recorded.

Add translated error info in data for error template. This is needed in order for theming error page with Twig.

For #458

bugfix: $langPath won't be defined if an exception is thrown by the method where we get its value from.

While parsing the XML metadata file of SP, attributes in AttributeConsumerService marked as required where added multiple time causing duplication of attribute keys in attributes.required section.

Add a couple of methods to perform translations with PHP's native gettext implementation, and start using them.

Since the class names were prepended with "\", the original extension was used, instead of our own wrapper. This fixes that issue, and sets the two new methods in SimpleSAML\Locale\Translate as the functions to use to replace the calls to gettext() and ngettext(). Support for the old translation system is still missing for twig templates.

Define SimpleSAML\XHTML\Template::$twig as a property (previously only set in the constructor) and make sure we use proper type checking when using it.

Use \SimpleSAML\Logger in the new Localization class, and fix general Exception being called from a namespace.

These two methods are equivalent to gettext/gettext's __() and n__(), but using strtr() instead of vsprintf(), so that the placeholders can be names instead of printf() format strings.

When we try to load a configuration file and there's a syntax error, PHP 7 will throw the new ParseError. We can take advantage of that to capture it and give the user a meaningful message back.

This resolves #483.

It is set later inside one of the branches of the if, but the other branches abort the execution of the method, so there's no need to define it outside the if.

This is related to #478.

This makes instanciating the Translate/Template-class unnecessary.

For #455

For #455