Add exec command callbacks for marathon events

Added exec command event callbacks so we can run shell commands like ```serf event marathon``` and send the data payload via STDIN, without requiring a listening HTTP server.

It feels a bit hacky in that its mostly a copy of the HTTP event code, but I think the entire event system needs a bit of refactoring anyway to make it more extensible.

To avoid breaking backwards compatibility I've left the HTTP endpoint for eventSubscribers alone and created a new endpoint for exec commands - ```eventSubscribersExec```.

It also currently blocks while executing the callback command.