Merge branch 'ci' into 'master'

ci/cd pipeline

See merge request !1

.gitlab-ci.yml

+---
+image: python:3.6
+
+stages:
+  - test
+  - build
+  - release
+
+lint:
+  stage: test
+  before_script:
+    - pip install flake8
+  script:
+    - make lint
+
+unittest:
+  stage: test
+  script:
+    - make test
+
+dist:
+  stage: build
+  script:
+    - make dist
+  artifacts:
+    paths:
+      - dist
+    expire_in: 1h
+
+release:
+  stage: release
+  only:
+    - tags
+  except:
+    - branches
+  before_script:
+    - pip install twine
+  script:
+    - make release