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_test.groovy

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