Issue#59

* Log is now appended by default (fixed issue #59)
* Logrotate is moved to SIGMINRT+10 signal (SIGUSR1-2 are used in GC)
* .pid and .lock files are added to use with logrotate and other
external tools. Corresponding options is added to pgator startup options. Default values are: `/var/run/pgator/pgator.pid` and  `/var/run/pgator/pgator.lock`.
* if pgator started with root previleges, try to drop them off when it
actually no need more. To do this pgator needs userid and groupid
specified in config (optional parameters).