Add logster output for Nagios Service Check Acceptor (nsca)

Added a logster output type (-o) of 'nsca', to allow logster to submit passive service check results to a Nagios instance. 
Requires the send_nsca utility to be installed to /usr/sbin/send_nsca on the hosts running logster

I've used this with success in a production environment to allow nagios to monitor services where the service status can only be practically derived from the existence (or not) of logfile entries.

The primary difference is that the 'metric value' being submitted should correspond to a nagios check return_code, so the parser in use must specifically support this. A metric.value of "0/1/2/3" corresponds to "OK/WARNING/CRITICAL/UNKNOWN" respectively.

This may be bending the paradigm too much to be merged in to the master.

NSCA Nagios addon details: http://exchange.nagios.org/directory/Addons/Passive-Checks/NSCA--2D-Nagios-Service-Check-Acceptor/details