INT-3250: Add @EnableIntegration infrastructure

JIRA: https://jira.springsource.org/browse/INT-3250

* The main purpose of this change to move all infrastructure bean definitions
from `AbstractIntegrationNamespaceHandler` to new `IntegrationRegistrar`.
* `IntegrationRegistrar` is invoked by standard `@Configuration` process and directly from `AbstractIntegrationNamespaceHandler`.
* Provide some refactoring for consistency.
* Add AnnotationContext tests

This change is some initial point for further work for SI Java DSL