Use static factory methods and proxyBeanMethods = false

It is better to be explicit about beans of type `Module` and make
sure they don't interfere with Spring trying to enhance
`@Configuration` classes. The tests are now clear of the pesky
warning logs, and there is a new section in the README.

src/test/java/org/springframework/guice/SuperClassTests.java

@@ -361,7 +361,7 @@ public class SuperClassTests {
 	static class DisableJITConfig {
 
 		@Bean
-		AbstractModule disableJITModule() {
+		static AbstractModule disableJITModule() {
 			return new AbstractModule() {
 				@Override
 				protected void configure() {