Periodic tasks refactor (#62)

* Once every second the session run a list of periodic tasks on every
instance concurrently. We use these tasks to do things like:
- Collect mem and cpu stats
- Check if instance is part of a swarm cluster
- Broadcast information to connected clients

services/task.go

@@ -0,0 +1,11 @@
+package services
+
+type periodicTask interface {
+	Run(i *Instance)
+}
+
+var periodicTasks []periodicTask
+
+func init() {
+	periodicTasks = append(periodicTasks, &collectStatsTask{}, &checkSwarmStatusTask{}, &broadcastInfoTask{})
+}