ratpack-thymeleaf: use guice multibinder to allow modules to register dialects (#253)

A simple module that uses multibinder to register a dialect looks like this:

```groovy
class ThymeleafLayoutModule extends AbstractModule {
     @Override
     protected void configure() {
        Multibinder.newSetBinder(binder(), IDialect).addBinding().to(LayoutDialect)
     } 
}
```