BridgeAbstract will return 3600 seconds by default, so the function
can be removed from any bridge implementing getCacheDuration
returning the same value.
Documentation updated accordingly.
Many bridges implemented getDescription() which is only applicable for
bridges extending RSSExpander.
Functions that are not part of the abstract class or interface should
be in private scope for better readability.
This fixes error "Using $this when not in object context"
Nested functions are not part of the object and therefore don't have
access to the object instance $this!
