Added CI github workflow and CD infrastructure

.github/workflows/continuous_integration.yml

@@ -0,0 +1,24 @@
+name: Continuous Integration
+on: [pull_request, push]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          token: ${{ secrets.PROGUARD_CORE_PERSONAL_ACCESS_TOKEN }}
+          path: proguard-main
+      - uses: actions/checkout@v2
+        with:
+          token: ${{ secrets.PROGUARD_CORE_PERSONAL_ACCESS_TOKEN }}
+          repository: Guardsquare/proguard-core
+          ref: github-workflow
+          path: proguard-core
+      - uses: actions/setup-java@v1
+        with:
+          java-version: 8
+      - uses: eskatos/gradle-command-action@v1
+        with:
+          build-root-directory: proguard-main/
+          wrapper-directory: proguard-main/
+          arguments: --include-build ../proguard-core/ jar