Add support for MIME-based message conversion

The MessageConverter interface in spring-messaging is now explicitly
designed to support conversion of the payload of a Message<?> to and
from serialized form based on MIME type message header.
By default, the MessageHeaders.CONTENT_HEADER header is used but a
custom ContentTypeResolver can be configured to customize that.

Currently available are Jackson 2, String, and byte array converters.
A CompositeMessageConverter can be used to configure several
message converters in various places such as a messaging template.