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/reactor.groovy

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