This is the initial commit

This plugin will allow people to keep track of Logstash availability by sending the output (a string, a counter, or an epoch timestamp) out to some other monitoring entity.  The default interval is 60 seconds, but it can be as low as 1 second.
@whack and I discussed it and felt that it was better having fewer options.  We can change it if that is what the consensus wants, but I am in agreement with @whack here on just one configuration flag with different options.  The generator plugin already does this.

I could put `"host" => @host` in the event creation. That will save a step.  This is a fairly lightweight operation, though, no more frequently than once per second, so readability and familiarity were the goal.