implement scheduler

- scan for apps every 10 minutes and schedules tasks
- uses docker.exec
    - there is no way to control exec container. docker developers
      feel exec is for debugging purposes primarily
- future version will be based on docker run instead

part of #519

src/addons.js

@@ -90,6 +90,12 @@ var KNOWN_ADDONS = {
         backup: NOOP,
         restore: setupSendMail
     },
+    scheduler: {
+        setup: NOOP,
+        teardown: NOOP,
+        backup: NOOP,
+        restore: NOOP
+    },
     simpleauth: {
         setup: setupSimpleAuth,
         teardown: teardownSimpleAuth,