Fully automate dev setup with Gitpod

This commit implements a fully-automated development setup using Gitpod.io, an
online IDE for GitHub and GitLab that enables Dev-Environments-As-Code.
This makes it easy for anyone to get a ready-to-code workspace for any branch,
issue or pull request almost instantly with a single click.

.gitpod.yml

@@ -2,5 +2,5 @@ image:
   file: .gitpod.Dockerfile
 
 tasks:
-  - init:  ./complete/mvnw package -DskipTests
-    command: java -jar complete/target/*.jar
+  - init:  cd complete && ./mvnw package -DskipTests
+    command: java -jar target/*.jar