AMQP-271 Allow Use of Standard CorrelationId Prop

Previously a custom header was used to convey
correlation data.

Provide the option to specify a header name, and
use the correlationId property by default.

Also, previously, to support "chained" application
modules, existing replyTo and correlation data was
pushed onto a "stacked" header and popped off when
receiving the reply.

This complexity was unnecessary because the
RabbitTemplate already maintains state for the
pending reply, so the existing replyTo/correlation
can be maintained there instead of adding it to
the message.

* Add Documentation
  - Add information to the what's new section
  - Add a subsection about message correlation to the main reference doc
Merge pull request #84 from garyrussell/AMQP-271

* garyrussell-AMQP-271:
  AMQP-271 Allow Use of Standard CorrelationId Prop