Add support of YAML

This commit migrates the YAML support available in Spring Boot to
the core framework. YAML documents can be loaded either as a
properties object or as a map.

Issue: SPR-9897
Additional tests for configuration class importing via ASM

Issue: SPR-11647
Eventual removal of deprecated AbstractBeanConfigurerAspect, BeanReferenceFactoryBean and CommonsLogFactoryBean
Fixed Xml/AnnotationBeanConfigurerTests after AbstractBeanConfigurerTests removal
Allow use of @SendToUser even w/o authenticated user

Before this change, subscribing to a user destination and use of
@SendToUser annotation required an authenticated user.

This change makes it possible to subscribe to a user destination from
WebSocket sessions without an authenticated user. In such cases the
destination is associated with one session only rather than with a
user (and all their sessions).

It is then also possible to send a message to a user destination
via "/user/{sessionId}/.." rather than "/user/{user}/...".

That means @SendToUser works relying on the session id of the input
message, effectively sending a reply to destination private to the
session.

A key use case for this is handling an exception with an
@MessageExceptionHandler method and sending a reply with @SendToUser.

Issue: SPR-11309
Configurable back off for listener recovery

Prior to this commit, DefaultMessageListenerContainer was recovering
on failure using a fixed time interval, potentially in an infinite way.

This commit adds an extra "backoff" property to the container that
permits to fine tune the recovery interval using a BackOff instance.

FixedBackOff provides a fixed interval between two attempts and a
maximum number of retries. ExponentialBackOff increases an initial
interval until a maximum interval has been reached. A BackOff instance
can return a special "STOP" time value that indicates that no further
attemps should be made. DefaultMessageListenerContainer uses this
value to stop the container.

protected method "sleepInbetweenRecoveryAttempts" has been renamed
to "applyBackOff" and now returns a boolean that indicate if the
back off has been applied and a new attempt should now be made.

Issue: SPR-11746
Add back-off attribute to JMS namespace

 This commit adds a "back-off" attribute to the jms:listener-container
 element so that a BackOff instance can be provided for users of the
 XML namespace.

 Issue: SPR-11746
Add BackOffExecution to isolate state

 This commit separates the BackOff configuration from an actual
 execution. BackOffExecution now contains all the state of a
 particular execution and BackOff is only meant to start (i.e.
 create) a new execution.

 The method "reset" has been removed as its no longer necessary:
 when an execution does not need to be used for a given operation
 anymore it can be simply discarded.

 Issue: SPR-11746
isLiteConfigurationCandidate considers @ComponentScan and @ImportResource as indicators as well

Issue: SPR-11769
Ensure handshake attrs are copied in SockJS session
Add WebSocket scope

This change adds support for a custom "websocket" scope.

WebSocket-scoped beans may be injected into controllers with message
handling methods as well as channel interceptor registered on the
"inboundClientChannel".

Issue: SPR-11305
JasperReports-related polishing
Compilation compatibility with JasperReports 5.5.2
Building against Groovy 2.3 instead of 1.8; updated several dependencies to latest maintenance releases (JasperReports 5.5.2, Tiles 3.0.4, Jetty 9.1.5)
Building against Quartz 2.2.1 and JavaMail 1.5.2
Undertow-related polishing
polishing

This commit adds an explicit error message when the BackOff instance
has returned BackOffExecution#STOP which basically means that no
further attempts are allowed.

This error message is a convenience way to notify that the container is
about to be shut down.

Issue: SPR-11746
Provide builder for ResponseEntity

This commit introduces a ResponseEntityBuilder, which allows for
building of a ResponseEntity through a fluent API.

Issue: SPR-11752
Fix RestTemplate documentation for gzip encoding

Prior to this commit, RestTemplate's documentation advised to a
DecompressingHttpClient decorator along with Apache's HttpClient in
order to support gzipped responses.

Since this is now deprecated as of Apache HttpClient 4.3+, this commits
update the documentation with HttpClientBuilder.
Polish