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.
Merge pull request #188 from mikereinhold/shutdown-hook-action

ShutdownHook Configuration  Element