Allow RESTEasy Spring integration under RESTEasy Spring Boot starter

We have created a RESTEasy Spring Boot starter and made it available as an open source project. Here it is.
It is functional and we will publish the first RELEASE version in a few weeks.

In order to develop RESTEasy Spring Boot starter, we had to get around a few things in RESTEasy, since we believe it was not designed to be used in an embedded container in Spring Boot.

The work around is in these three classes:

1. CustomDispatcherResteasyDeployment.java
2. CustomListenerBootstrap.java
3. CustomRegistrySyncronousDispatcher.java

And these three work around classes get used in method resteasyBootstrapListener in class ResteasySpringBootConfig.java.

Ideally, RESTEasy classes ResteasyDeployment.java and SynchronousDispatcher.java would have to be adapted (small and concise changes), so those three work around classes could be removed, and the integration between the starter and RESTEasy would be better.

So, we are suggesting these changes actually in this PR.
Please let me know if you have comments or questions.