add drone config to build and push artifacts

.drone.yml

@@ -0,0 +1,20 @@
+pipeline:
+  build:
+    image: node
+    commands:
+      - npm install -g bower gulp
+      - npm install
+      - npm test
+      - gulp
+    event: push
+
+  update:
+    image: appleboy/drone-git-push
+    branch: master
+    commit: true
+    commit-message: "[SKIP CI] update artifacts"
+    force: false
+    secrets:
+      - DRONE_COMMIT_AUTHOR
+      - DRONE_COMMIT_AUTHOR_EMAIL
+      - GIT_PUSH_SSH_KEY