Documentation improvements many of which have been proposed by Anton Tagunov.

Add references to SLF4J in the manual, so that new users have an easier time to find it.

  Chapter3 has been split into two parts, the fist part on logback configuration and a second part on Joran as a standalone framework

- ExecutionContext class has been renamed as InterpretationContext a few months ago. Syncing the docs.
- ongoing work on the documentation

  The previous syntax is still honored but it will no longer be documented.

- ongoing work on the docs

really skip over children when an error occurs. In the previous version of the code, 
we would skip over invoking the action but would still try to find actions for the event. 
In this revision, we skip over the event as a whole, in particular we no longer look up
actions for it. In practical terms, this just avoids adding error statuses for not found 
implicit actions. 

Since we now only support skipping children, removed the SkipCode enumeration from the code.

Added test cases making sure that the bug was corrected.

Simplifying the Joran's logic in case of errors within actions. 
In case of a failing action, Joran skip the children of the event 
associated with the action. In practice, this was already the case. 
However, with this commit, we are formally dropping support for 
SKIP_SIBLINGS.

Test cases will follow

  other modules, i.e. logback-classic

- FruitShell code is intended to test Joran's replay capability. 
  Thus the various config files have been moved under src/test/input/joran/replay
  

  It's static methods have become instance methods with their 
  first argument (a LoggerContext) removed. This leads to very slightly
  simpler code.

- added chapter numbering by CSS (firefox only)

- simplification of LoggerStatusPrinter
- improvements to documentation

Given that GPL+classpath exception is rejected in the same way as LGPL
by those requesting for a licensing change, there is no need for
change if the end result is the same.

- unit testing addition of status listener by logback.statusListenerClass system property

- Setting the logback.statusListenerClass system property to the name of a StatusListener 
  class will automatically instantiate an instance of that class and register will the 
  StatusManager currently in use by the logger context. By convention, SYSOUT represents 
  a shorthand for ch.qos.logback.core.status.OnConsoleStatusListener

- documentation to follow