Polish formatting and update links in README
Link to @SpringCentral in README
update org name in links

SpringSource github org name changed to spring-projects.
Update link to Pro Git in CONTRIBUTING page
Update link to Pro Git in CONTRIBUTING page
Add marshalling hooks in Jaxb2RootElementHttpMessageConverter

Allow Jaxb2RootElementHttpMessageConverter subclasses to customize
the {@link Marshaller} and the {@link Unmarshaller} created by the
message converter.

Issue: SPR-11488
Move customize(Un)Marshaller methods to abstract class

Issue: SPR-11488
Merge pull request #474 from sdeleuze/SPR-11488
Add Servlet 3.1 methods to mock request

Issue: SPR-11492
Added explicit note on thread safety to JdbcTemplate variants

Issue: SPR-11478
Added documentation on how to set up JSR-303 method validation

Also removed outdated Struts references and made use of 'javadocs' term consistent.

Issue: SPR-11473
Update ref. manual to favor constructor injection

The breakout box named "Constructor-based or setter-based DI?" in the
reference manual currently recommends the use of setter injection.

This commit refines this text to align with current best practices and
now favors constructor injection over setter injection.

Issue: SPR-11459
Merge pull request #466 from olivergierke/SPR-11459

* SPR-11459:
  Update ref. manual to favor constructor injection
Fix DocBook to AsciiDoc conversion errors

 - "__ `text`__" --> "__`text`__"

 - "`text` s" --> "`text`s"
Add smart logging for disconnected SockJS clients

The Servlet API does not provide notifications when a client
disconnects, see see https://java.net/jira/browse/SERVLET_SPEC-44.
Therefore network IO failures may occur simply because a client has
gone away. Before this change that could fill logs with unnecessary
stack traces.

After this change we make a best effort to identify such network
failures, on a per-server basis (tested with Jetty, Tomcat, Glassfish,
and WildFly), and log them under a separate log category.

A simple one-line message is logged at DEBUG level (i.e. no stack trace)
while a full stack trace is shown at TRACE level.

Issue: SPR-11438
Polish AbstractSockJsSession
Clarify exception handling in converter

Issue: SPR-11500
Update reference documentation for SPR-11486

Issue: SPR-11486
Outdated code example correction in section 10.3.6
Merge pull request #476 from platan/master

Update MockRestServiceServer example in reference manual