Moved an executor aspect to a bean post processor

without this change when an executor implementation was a final class or had a final method - the application wouldn't start or would produce NPE at startup.
    with this change we have a custom bean post processor that wraps an aspect manualy either via a JDK Proxy when a class is final or a method is final or creates a CGLIB proxy if that's possible.

    fixes #550

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-websocket/pom.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!--
-  ~ Copyright 2013-2016 the original author or authors.
+  ~ Copyright 2013-2017 the original author or authors.
   ~
   ~ Licensed under the Apache License, Version 2.0 (the "License");
   ~ you may not use this file except in compliance with the License.