Setup Gradle multi-module build

Added core, config and samples modules. Moved sample project into samples module.

Closes gh-10

samples/helloworld/src/test/java/sample/ApplicationTest.java

@@ -0,0 +1,16 @@
+package sample;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.ApplicationContext;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@SpringBootTest
+public class ApplicationTest {
+
+    @Test
+    public void loadContext(ApplicationContext context) {
+        assertThat(context).isNotNull();
+    }
+}