Add support for network events in Listen 2.5

Listen 2.5 now supports [receiving filesystem change events over the network](https://github.com/guard/listen#forwarding-file-events-over-tcp). I've added a `-o / --listen-on` option to guard which makes use of these events (e.g. in Vagrant VMs which have problems with native guest filesystem events).

I'm not totally happy about the basic `listen` script I've used on the host (see README). It works but can't be killed from the console. Perhaps someone could write a better version which could be shipped with guard or listen?