LOGBACK-833 Fixes for variables in sifting appender

Here are fixes for:
 - http://jira.qos.ch/browse/LOGBACK-833
 - http://jira.qos.ch/browse/LOGBACK-835

Main changes
 * One can now use <timestamp/>, <variable/> or <property/> inside a SiftingAppender.  
 * <timestamp/> has been made to default to setting a local property
 * Appenders with SiftingAppender can now refer to local properties of the enclosing scope.