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
INT-3250: Polishing

* Remove `@EnableIntegration#defaultPublisherChannel`
* Revert `#jsonPath` & `@xpath` registration logic
* Fix for DEBUG message on each parse

JIRA: https://jira.springsource.org/browse/INT-3258
INT-3250 Fix JavaDocs & IDEA+Gradle srcDirs issue
INT-3250: Revert `build.gradle` changes