add ResourceExistsPropertyDefiner
Introducing AbstractDiscriminator base class to avoid repeated start()/stop() implementation in each concrete Discriminator
Support attribute "optional" in include element

Support attribute "optional" in include element to prevent errors on
inclusion of non-existent files without requiring Janino and
FileExistsPropertyDefiner.
refactoring CyclicBufferTracker and AppenderBufferTracker into the more generic ComponentTracker
test improvements
builds cleanly
fix logback-767

 StatusListener has two instances OnConsoleStatusListener and instance by
 seted classname.
 The second be Judged that is ContextAware and LifeCycle's implement and start it.
 but The first is not to Judged.

 all StatusListener must be Judged and start it.
AppenderTracker is now extends AbstractComponentTracker, removed support for sifting in Gaffer, tests pass
tweaking tracker related changes
more tracker refactoring, unit tests to follow
fix LOGBACK-845, CyclicBufferTracker_TImpl renamed as CyclicBufferTrackerT
fix LOGBACK-846
more refactoring, added unit tests
make all tests pass
make the AppenderFactory related code in c.q.logback.core.sift suck less
more unit tests
more unit tests
allow timeout to be specified as a duration
updates to documentation
Merge pull request #115 from bjhanfeng/logback-767

fix logback-767