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 a new `IntegrationRegistrar`.
* `IntegrationRegistrar` is invoked by the standard `@Configuration` process and also directly from `AbstractIntegrationNamespaceHandler`.
* Provide some refactoring for consistency.
* Add AnnotationContext tests

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

INT-3250 Polishing

* Remove commented out code
* Add javadoc to IntegrationRegistrar.registerBeanDefinitions()
* Fix javadoc links to other Spring projects (and JVM, JEE)
* Add reference documentation and what's new