New doxygen stuff checked in for client API

[Jonathan Robie <jo...@redhat.com>]

Alan check in the new doxygen code for me.

Doxygen now looks different and has more info.

There's a general overview on the main page, with code snippets, and 
code snippets for the major tasks associated with the Session, Message, 
MessageListener, and SessionManager classes. The inline docs in the 
project are also preserved, of couorse.

Let me know if something needs fixing, or if you have documentation to 
suggest for other classes.

Jonathan