exclude the purge dirs
added purge dirs

idea by Rob van der Wal
moved code to subroutine
use listdir() for efficiency
use a callback function for looking under purge/
diff works on files under purge/
purge single files
added exec_command(cmd_arr)
client will purge files rather than master
purge: guard against user mistakes

Do not allow files in the purge root dir; it would likely destroy the
entire system
do not require purge/all/
add purge dirs to rsync filter