ENH: Nagios filter

Nagios is a monitoring system, where the remote Nagios server hosting the user interface, connects over TCP to the nrpe2 (Nagios Remote Plugin Execution) daemon on the port 5666 by default. If the port is not closed on the firewall to everyone except the Nagios server, fail2ban can detect and ban any intrusion attempts by parsing the syslog (on FreeBSD, the nrpe2 daemon logs to /var/log/messages).

PS: this is my first full GIT pull request, and although I tried to do everything right, I certainly did not. I am not at all familiar with GIT. I am also not sure whether I am supposed to run the coverage script for filter changes too. I installed the py-coverage package, and tried running the commands as described in the DEVELOP document, but it spits too many cryptic errors on me, that do not seem to have anything to do with the changes I did. However, I am using the nagios filter since a while, and it works fine.