- FileNamePattern now slashifies its pattern argument at construction time

  This eliminates the need to slashify regular expression at a later time 
  (since the original pattern is already slashified)
- While cleaning archived log files, empty fodlers are taken into account, especially if
  they are in cronolog-style "/var/tmp/2009/08/04/foo.log". 
- old archives are cleaned for time-based triggering as well as size-and-time-based triggering
- relevant test cases