Make Message type pluggable

To improve compatibility between Spring's messaging classes and
Spring Integration, the type of Message that is created has been made
pluggable through the introduction of a factory abstraction;
MessageFactory.

By default a MessageFactory is provided that will create
org.springframework.messaging.GenericMessage instances, however this
can be replaced with an alternative implementation. For example,
Spring Integration can provide an implementation that creates
org.springframework.integration.message.GenericMessage instances.

This control over the type of Message that's created allows messages
to flow from Spring messaging code into Spring Integration code without
any need for conversion. In further support of this goal,
MessageChannel, MessageHandler, and SubscribableChannel have been
genericized to make the Message type that they deal with more
flexible.
Add MessageBuilder