Separate device functionality into plugins.

lib/roles/device/plugins/heartbeat.js

@@ -0,0 +1,20 @@
+var syrup = require('syrup')
+
+var wire = require('../../../wire')
+var wireutil = require('../../../wire/util')
+
+module.exports = syrup()
+  .dependency(require('./push'))
+  .define(function(options, push) {
+    function heartbeat() {
+      push.send([
+        wireutil.heartbeat
+      , wireutil.envelope(new wire.DeviceHeartbeatMessage(
+          options.serial
+        ))
+      ])
+      setTimeout(heartbeat, options.heartbeatInterval)
+    }
+
+    heartbeat()
+  })