GH-238 Added initial support for RoutingFunction

- Added initial implementation of RoutingFunction which is bootstrapped optionally based on setting ‘spring.cloud.function.routing.enabled’ property to true.
- Added initial documentation and tests

Resolves #238

spring-cloud-function-core/src/main/java/org/springframework/cloud/function/core/WrappedFunction.java

@@ -35,7 +35,7 @@ import org.reactivestreams.Publisher;
  * @author Oleg Zhurakousky
  * @since 2.0.1
  */
-abstract class WrappedFunction<I, O, IP extends Publisher<I>, OP extends Publisher<O>, T>
+public abstract class WrappedFunction<I, O, IP extends Publisher<I>, OP extends Publisher<O>, T>
 		implements Function<IP, OP>, FluxWrapper<T> {
 
 	private final T target;