Ensure CLI adds @EnableAutoConfiguration in an appropriate place

Up to now we have been treating the *first* class to be compiled
as the "main" application and adding @EnableAutoConfiguration. This
isn't always appropriate (e.g. if it's a test case), so now we
look for an appropriate annotation before falling back to the old
behaviour.

In addition ensures classes with a field of type Reactor trigger
the reactor auto imports.

See gh-969

spring-boot-cli/test-samples/integration_auto.groovy

@@ -0,0 +1,19 @@
+@SpringApplicationConfiguration(classes=Application)
+@IntegrationTest('server.port:0')
+@WebAppConfiguration
+class RestTests {
+
+	@Value('${local.server.port}')
+	int port
+
+	@Test
+	void testHome() {
+		assertEquals('Hello', new TestRestTemplate().getForObject('http://localhost:' + port, String))
+	}
+
+	@RestController
+	static class Application {
+		@RequestMapping('/')
+		String hello() { 'Hello' }
+	}
+}

spring-boot-cli/test-samples/integration_auto_test.groovy

@@ -0,0 +1,13 @@
+@SpringApplicationConfiguration(classes=ReactorApplication)
+@IntegrationTest('server.port:0')
+class RestTests {
+
+	@Autowired
+	Reactor reactor
+
+	@Test
+	void test() {
+		assertNotNull(reactor)
+	}
+
+}

spring-boot-cli/test-samples/reactor.groovy

@@ -0,0 +1,4 @@
+@Configuration
+@EnableReactor
+class ReactorApplication {
+}