added retention rules to piler core

etc/example.conf

@@ -11,6 +11,12 @@ verbosity=1
 ; it it was started by root
 username=piler
 
+; by default (unless a specific retention rule applies),
+; preserve an email for this many days. The purge utility
+; won't harm the message until its retention days are gone.
+; The default is 7 years + 2 days (=7*365+2=2557 days)
+default_retention_days=2557
+
 ; this is a 16 character long vector
 iv=****************