Integrate a basic gitlab-CI specification for maven builds

Signed-off-by: Alexander Diewald <diewald@fortiss.org>

.gitlab-ci.yml

+image: maven:latest
+
+stages:
+  - build
+  - test
+
+variables:
+  GIT_SUBMODULE_STRATEGY: recursive
+  MAVEN_CLI_OPTS: "--batch-mode"
+  MAVEN_OPTS: "-Dmaven.repo.local=.m2/repository -Dbuild.tooling=true -Dbuild.af3=true -Dbuild.emf=true"
+
+cache:
+  paths:
+    - .m2/repository/
+    - target/
+
+build:
+  stage: build
+  script:
+    - mvn $MAVEN_CLI_OPTS verify
+
+test:
+  stage: test
+  script:
+    - mvn $MAVEN_CLI_OPTS test