Example device plugin and helpers

This PR introduces:
* An example device plugin that exposes files as devices for testing.
* Helpers for serving a device plugin
* A launcher binary that allows interacting with a device plugin without
needing a Nomad client.

plugins/device/cmd/example/cmd/main.go

@@ -0,0 +1,18 @@
+package main
+
+import (
+	log "github.com/hashicorp/go-hclog"
+
+	"github.com/hashicorp/nomad/plugins"
+	"github.com/hashicorp/nomad/plugins/device/cmd/example"
+)
+
+func main() {
+	// Serve the plugin
+	plugins.Serve(factory)
+}
+
+// factory returns a new instance of our example device plugin
+func factory(log log.Logger) interface{} {
+	return example.NewExampleDevice(log)
+}