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.
I think it would make the function easier to test if you extract the `Event` creation from the loop into his own method.

I find it confusing to check the content of the message to create a new kind of event, would it be better to have some sort of `type` option? This will allow use to do some validation on the value.