webhook plugin for ping events

This is a generic plugin for sending ping events as JSON to an external HTTP service. This is probably more educational than practical at this point, but I can definitely see using this as a way to submit events back into the metrics/event-stream (e.g. via https://github.com/obfuscurity/backstop) or to message routing services (on my TODO list).

Sample output from this plugin:

```
POST /custom HTTP/1.1
Host: 127.0.0.1:5001
Connection: keep-alive
Transfer-Encoding: chunked

0

d1
{"__v":0,"monitorName":"origin","check":"507d54fcae02010000000007","time":134,
"isResponsive":true,"isUp":true,"_id":"507d67f1b5cc990000000001","tags":
["graphite","good"],"timestamp":"2012-10-16T13:58:09.656Z"}
fd
```