[test] add CI support

- Resolves: #13

.gitlab-ci.yml

+---
+stages:
+  - build
+
+variables:
+  GOPATH: /go
+  APP_PATH: /go/src/0xacab.org/leap/bitmask-systray
+
+build_test:
+  image: 0xacab.org:4567/leap/bitmask-dev:latest
+  stage: build
+  script:
+    - apt-get -y install libappindicator3-dev libgtk-3-dev libzmq3-dev golang
+    - mkdir -p /go/src/0xacab.org/leap/
+    - ln -s "$(pwd)" ${APP_PATH}
+    - cd ${APP_PATH}
+    - go get .
+    - go test ./...
+    - go build
+  tags:
+    - linux
+  artifacts:
+    paths:
+      - 'bitmask-systray'
+    expire_in: 1 month