ShutdownHook Configuration  Element

Added configuration framework for having Logback automatically stop its
context via a shutdown hook. Enabled via the shutdownHook configuration
element (backed by the Joran ShutdownHookAction). 

Basic implementation provided, DelayingShutdownHook, stops the Logback
context after a specified delay (default is 0 for immediate shutdown).

Updated news page under revision 1.1.3 and added details to the
configuration manual page.