INT-1983 - Add JPA Adapter

For reference see: https://jira.springsource.org/browse/INT-1983

* INT-2435 - Add the ability to automatically resolve the entity-class from the payload as fallback
* Add <transactional/> support for **Jpa Outbound Gateway** and **Jpa Outbound Channel Adapter**
* Merging Documentation from: https://github.com/amolnayak311/spring-integration/blob/INT-2440/src/reference/docbook/jpa.xml
* Provide JPA Tests against Hibernate, EclipseLink and OpenJPA
* Improve Documentation
* Remove trailing white-space, convert white-space to tabs, remove @transactional from DefaultJpaOperations
* INT-2440 Adding the documentation for the JPA adapters

INT-1983 - Code Review based Fixes
* Fix indentation in XML Schema
* Remove *transactional* sub-element definition from JPA Inbound Channel Adapter in XML Schema
* Fix enumeration *gatewayType*

INT-1983 - Based on Code Review

* Remove '/** {@inheritDoc} */' from DefaultJpaOperations
* Remove JavaDoc parameters that don't contains details
* Cleanup *JpaUtils*
* Remove "transaction-manager-ref" from *spring-integration-jpa-2.2.xsd*
* Cleanup *spring-integration-jpa-2.2.xsd*
* Provide documentation to *EclipseLinkJpaOperationsTests* on how to run tests in IDE
* Provide documentation to *OpenJpaJpaOperationsTests* on how to run tests in IDE
* Refactor JPA Reference Doc Chapter (work in progress)

INT-1983 - Code Review: provide better JavaDoc

src/reference/docbook/index.xml

@@ -140,6 +140,7 @@
       <xi:include href="./http.xml"/>
       <xi:include href="./ip.xml"/>
       <xi:include href="./jdbc.xml"/>
+      <xi:include href="./jpa.xml"/>
       <xi:include href="./jms.xml"/>
       <xi:include href="./mail.xml"/>
       <xi:include href="./mongodb.xml"/>